Ive been playing this with friends since it arrived on steam, and im actually really liking it so far. I did download it at one stage before and only dabbled with it solo, but now im playing through with premade and pug groups which makes it much more enjoyable. One surprising aspect of the game is the challenge; for a cutesy, simple looking game it does at times pack quite a punch. The arena challenge rooms are a survival type of game, and things get pretty frantic.
The mist system is a bit hit and miss IMO, but you do get a free mist tank after the tutorial (which i'd recommend saving) which refills your energy to 100%.
You get 100 mist energy each day, and to travel to each floor of the dungeon it costs you 10. You can also use mist to revive on death, but its not recommended as other players can revive you in battle and the energy costs get very steep.
I spent £3.05 (approx 5 USD) which allowed me to buy 1600 extra energy, which is going to last me weeks. Id say if you enjoy the game, put $10 aside and buy yourself some mist, for that money you would end up having over 3k which is going to take an absolute age to get through unless youre playing for hours and hours a day. I do wonder how bad reliance on cash shop mist is going to be at higher stages though.
YouTube - ‪Spiral Knights Gameplay - First Look HD‬‏
Spiral Knights - MMO Hut