I Spent a good portion of this weekend playing Hawken (mostly straight deathmatch, I'm pretty terrible at team games.) and I've been quite enjoying it. Once I got out of the CR-T Recruit I switched to an Assault (all-around medium class) and did reasonably well with that, but so far I seem to be having the most success with the Reaper (light class sniper,) and I can even manage to win a couple of rounds here and there. Given the fact that I'm generally terrible at shooters (especially on consoles) this is a bit of an accomplishment for me.
In theory you could get all the mechs and all the upgrade parts without paying for anything, but you'd have to spend an awful lot of time playing to do it (you generally get about 150HC per match, and mechs cost about 6400HC, while upgrade parts cost between 1500-5000HC apiece.) All of the cosmetic upgrades, on the other hand, require paying for them, but I ended up finding one of the NVidia $50 in-game credit codes on eBay for $15, and I figure that should be enough to last a while. Normally I'm not big on the whole Freemium model, but if a game is good enough I don't have a problem spending some money on it to support the developers, and I think that's the case with this one.