Spiral Knights is a game with massive potential that the developers can't seem to act on. The only promise of new content right now is a gunner update that never seems to arrive, and it's unlikely it will change anything.
Weapon and game balance issues are just the beginning of Spiral Knight's problems. Spiral Knights has a severe lack of content. This is partially disguised by the fact that you will have to grind for hours, days, and even weeks just to get some decent gear (or just pay up, of course, what did you think "free-to-play" meant?).
The developers have a horrible habit of releasing cosmetics and rehashing old events. There are a few events that offer a few non-cosmetic items, and these return semi-periodically to drag experienced players back into the game.
As I said, the grinding in Spiral Knights is severe. The game starts off slowly, but that's nothing compared to the late-game. To make one good item, you have to get 3 orbs (whether that be through payment or by actually playing the game, and neither option is efficient), make a low-tier item, use it for a while, then get 3 more orbs (that are much harder to get), use the new resulting item for a much longer while, then get another 3 orbs (which are actually much easier to get, thanks to excellent game-balance), then you will finally have one item out of a set of multiple items. It can take a few days to get just one item. Getting a useful amount of items is just plain painful. Or you can give the developers your money. Either way works, right?
I might be willing to forgive the grinding if the gameplay fully made up for it. It does not. It's fun at its core, but extremely repetitive and unbalanced. There are a few enemy types, and each type only has a handful of enemies. The fiend type is stupidly OP, and the beast type is stupidly UP.
And as for level balance, it's not much better. Almost any Spiral Knights player will shiver at the word "compound." Levels are somewhat randomized, meaning the room layouts and status debuff types are different. The debuffs and enemy types just change what kind of armor and weapons you'll use (making grinding all the more necessary in order to get a variety of items), and while room layous are differed, the rooms remain the same. Some rooms are more rare than others, and sometimes rooms won't appear at all, but all in all, you'll really start to notice how similar all levels seem once you've played for a while. Not to mention there are plenty of levels that aren't even randomized. Considering the boss levels (the levels you're most likely to grind, because they have the best loot) fall into this category, that means the game becomes very, very repetitive.
As for end content, the game has very little. For a considerable fee, you can get one try at fighting an end-game boss. The only real way to prepare for these bosses is to bring friends (going on your own is pretty much suicide), and wear a certain type of armor. As if that's not enough, you'll have to beat them multiple times to get certain gear. Oh yeah, did I mention the end-game bosses are just rehashes of the game's other bosses? Well, they are. And this isn't a Terraria-style rehashing, where they actually feel and play differently.
If I do have to give some points to the game, it's for music, art, and guilds. The game has some pretty good music. Unfortunately, you'll be hearing it for extended periods of time. As for art, the developers are constantly adding cosmetics instead of actual content, so you know they have to be at least somewhat good in that field. As for guilds, Spiral Knights allows you to very conveniently meet other players and play with friends in the form of guilds. Of course, making a guild costs in-game points, but once you get past that, they're pretty good. You can even colaborate with your guild to make and decorate your own guild-hall, and visit other guild's halls.
Overall, I can't recommend Spiral Knights. I feel like the developers don't know what they're doing. They try to use grinding as a source of money, but that just ends up pushing away potential new players (as if the slow start of the game wasn't enough). The game needs content badly, and it's just not getting it. If there's one thing that really kills the game, it's the grinding. A lot of the items and weapons in the game are founded on some unique ideas, but they take way too long to get, and the balancing is just bad. At its core, Spiral Knights is made from good ideas, and it could be fun, but it simply won't realize its full potential.
TL;DR don't play, definitely don't pay