First off, buy the BASE GAME + the GATHERING OF POWER expansion. It includes all the features and content of the other two DLCs and it's a very naughty move on part of the developers to still sell the older two expansions on Steam, potentially making you pay $30 to buy all the expansion content, when you can be buying it for $10.
With this unpleasant situation out of the way and despite it, I still highly recommend the game. It is co-developed by Richard Garfield of Magic: The Gathering and Netrunner fame. The game uses many concepts similar to MTG but is presented in a more casual framework. The major differences from MTG are: a) no card draw or deck but a set of semi-random given cards that don't 'run out' and are always available to cast if you have the mana, b) slot-based battlefield with two lines of 6 slots in which players summon creatures.
Spectromancer is far easier to pick up and play compared to MTG. As there is no deck building, gameplay offers more limited options but the game livens things up with its unique class system, with each class getting its separate set of cards. Creature interplay is far more important and summoning order and spell support often require as much strategy as a game of MTG.
Multiplayer, a very big factor for Spectromancer's appeal, is still alive and well, with available players at all times and lots of replayability given the large number of classes available.