TL;DR: It's awesome if you know what you're getting and if you're into this kind of game. To know what you get I guess you have to read the rest of this wall of text. I promise it will be a nice and informative read while covering most of what you'd want to know before you buy the game or one of the two DLC's.
Unity of Command is an excellent game that manages to achieve a thing rarely achieved by wargames that deal with WWII on the operational level: It's accessible to newcomers and easy to learn, it's simple on the surface and in its gameplay but it still has enough depth and complexity to offer interesting decisions for veteran wargamers, because almost every decision you make carries some weight when it comes to the end result. There are other wargames comparably excellent to this one, but they make most newcomers want to vomit blood out of their eyes - because the developers place their confidence in you being willing to read a 100 page manual alongside playing their game and they assume you will look past the hideous and needlessly overcomplicated user interface. (I'm looking at you Korsun Pocket - excellent game, terrible learning experience).
None of these things are an issue in Unity of Command. The user interface is excellent and very user-friendly. You don't have to click on a unit to get information about whether it can still attack or move, you see it at a glance because each unit has tiny icons below and around it, that tell you everything you could possibly want to know about your or your enemies' unit at a glance without wasting your time and clicking every unit to check manually.
Another point where this game shines is its "realism". I know this word is hard to swallow if you're a "serious" wargamer and you look at Unity of Command. But this game basically models the higher-level essential things of what war in the eastern front at the operational level was all about. For the Germans it was about advancing as fast and as far as you can to take key objectives without outrunning your own supply or your supply lines being cut off by the enemy. The flipside of this naturally means encircling your enemy and cutting of their supply to weaken them and eventually starve them out completely. Also it's about breaking through (sometimes multiple) defensive lines of your enemy. Pick the weakest link in their line, punch a hole with a massed attack at that point, and then squeeze through everything that has wheels to advance as fast as possible towards your objectives, while your slower infantry divisions advance behind your motorized divisions and worry about securing the supply line for those advancing units, whilst simultaneously trying to encircle and destroy the enemy. The deceptively simple game mechanics of movement and attack (whilst also considering rivers, terrain features and weather effects on your movement and attacks) are excellent at modelling all of the above - and in the end I think the gameplay mechanics really model everything truly relevant in the context of this large scale warfare level.
Each of the units you'll see in the game represents several thousand, if not ten-thousand men and each single single hex represents 20 kilometers. Almost all you ever do in this game is move and attack with your divisions, move and attack. But where to move? What to attack? What to attack first and what with? Where to break the enemy line? How do you cut off their supply lines to starve them out? You also make important decisions in the form of "theatre assets" (air raids, building or blowing up bridges, extending the range of your supply depots etc.) Also you have to decide about reinforcements as the game progresses over turns (each turn represents 4 days), so new reinforcements may come into play - on your side as well as on the side of your enemy. Sometimes you need to take a specific area at the edge of the map so your own reinforcements may enter from there, at other times you just replace some losses that your divisions in play suffered already (which sensibly you can only do if they are in supply).
So yes at first glance your options seem somewhat limited but they always make perfect sense in the context of modelling the war in the east. You could bomb a city where an enemy infantry division has entrenched itself, but each air attack you launch on that city has a chance of turning the city into ruins, giving any enemy division within it even more defense as fighting on a pile of rubble increases their chances to hide and ambush if you launch an attack into that city. Some divisions have artillery attached, but if you move that unit its artillery attachment ("step") won't contribute to your attack value, it will only contribute in your defense and attack after you end your current turn and they have set up. This game is just full of gameplay details like this which make absolute sense. If you enjoy pondering these kinds of decisions with a feeling that virtually each small decision matters for the outcome, then this is a game for you. You can also play this game okay without paying much attention to the deeper details - you won't excel at it but you could still play very competently and there aren't that many details to know either, the complexity of your options and decisions arises from the interaction of the gameplay mechanics and the finer details.
Also there are "Zone of Control" mechanics meaning every (non-weak) unit in a hex extends a zone of control into all six hexes around it (as long as those hexes are inside the own territory), meaning enemy divisions who move into these hexes are immediately "pinned down" by the division controlling the zones around it and so the pinned unit can't move any further this turn. This leads to excellent tactics, where some of your units "clear" those zones of control and get pinned, while others advance through and past those friendly pinned divisions and thus can advance deeper into enemy territory without getting pinned themselves. You'll learn what that means and how important zones of control are soon enough, because the nothing short of excellent AI will constantly make intelligent use of this mechanic to exploit any hole and weak link in your own defensive lines which may have looked good and solid enough a second ago when you ended your turn, but proved to be as fragile as a butterflies wing a moment later. Also the AI is excellent at defending any objectives you need to take. It knows exactly what your objectives are and dynamically and competently adapts its defensive lines to hinder your progress at every turn. Good times.
I do have some gripes about Unity of Command as well however, but they are nowhere seriously bad enough to spoil the fun and experience you can have with this game. I've come across at least two maps that are ill-conceived for 1920x1080 resolutions, because there is a limit to which point you can drag the map view around and some of your user-Interface elements can end up blocking a part at the edge of a map that isn't greyed out and where the actual gameplay still happens (though you can quickly enable and disable user interface elements with hotkeys, so the problem is merely annoying, not gamebreaking). Also there is little variety in terms of music, but the music that is there is really nice. (And it's not as if I can't mute it and play my own music anyway).
If I had to criticize gameplay I would say it's mainly this: Your performance is solely ranked based on how timely you take objectives and nothing else. Rarely this can lead to stupid outcomes where I took Stalingrad on time with two completely weakened divisions, my other forces virtually obliterated. I'd have lost on the next turn, but because I took everything on time it was a "brilliant victory". Troop loss should have been included in your performance rating. Also random outcomes and weather can prevent you from achieving the best possible rating.