I played this quite some time ago, perhaps a year or two ago actually, somewhat briefly. It's a very interesting shooter, much different from any other shooter I'm aware of. I'm not really interested in giant mecha, but MechWarrior seems to get it right in my eye as opposed to the anime style that's so popular with giant mecha. It's surprisingly tactical, and wrapping your head around how movement works and how your aim is separated from the way you walk is much more akin to driving a tank than floating a camera around like in most FPS.
The main downside it, last time I recall, it's fairly Pay 2 Win, but that doesn't necessarily mean it isn't Free to Enjoy. It uses a system where every week(?) they swap out the available free mechs so you can try different stuff out before you lay any money down on it.
The way customization works is also very interesting, as you can see in the links in LoSboccacc's post. Your mech has slots in different regions of the body where you can place modules and weapons and the locations of these are actually rather important. Put a laser on your torso and you can only aim using your torso, while putting it on your arms means that you get better range of motion and can aim easier, but the arms typically can't hold the heavier weapons. You also have to be careful of heat, as your mech can overheat and be forced to power down in order to cool off which is extremely bad. You can slap heatsinks on different parts of your mech, but the locational damage means that if you put all your heatsinks into your left arm and the enemy shoots your left arm off (literally, this can happen) then all your heatsinks are gone. Be mindful of your targetting display and aim wisely, headshots may actually not be the end-all be-all strategy you find in many other shooters.
For example, on the mech shown above you might decide to aim for the right side of the mech (the left side from your perspective if you're face to face) since his right arm has more weapons and the right shoulder has a weapon while the left side of the body only has one weapon on the arm. If it's a fast mech you might aim for the legs to impair his movement.
Ah yes, the map you play on has different effects on certain things. Play on a cold map? Your mech heats up slower. Play on a hotter map, perhaps with steam vents? Your thermal vision gets blocked by the heat. Probably the most authentic thermal vision I've seen in a game
Can't remember much else about it, but it's free to play, so the only thing you're paying is your ISP for downloading it, basically.