I only got to play a few games last night before my farming partner in PoE dragged me away, but I have some first impressions as I only really played the raider (my rating reflects using a completely new mech, gonna have to break out the 25s soon to keep it over 1600 -.- I'm such a scrub lol)
Reflak is interesting, it's still a medium range at best weapon, but I really like the ricochet mechanic, and I think in time, it could really add to the game with it's ability to hit targets behind cover with poke damage.  It's heat feels about right, spam it too much and you over heat, coordinate burst fire with teammates will net you some nice assist exp.  Sadly I only got to level 3 so I didn't have time to work out things with the charge flak.
Corsair (disclaimer, I really, really, like this weapon)  
RPG mode:  I keep wanting to air detonate it, that's literally my only gripe.  It's so damn gratifying getting full hits/kills with this mode, it's like christmas, damage feels just a might low, but then again I missed at least 75% trying to longbow people behind cover/on top of AA in origin (I'm looking at you [HWK] Defty, you will come to fear my snipe RPGs >

)
MIRV (I call it the Boomstick, cause thats what it is.): People are gonna cry this is OP all damn day.  People are going to cry that it needs more range, or should have remote detonation, etc etc.  This mode of the weapon is, freaking amazing.  It's a great finisher for that last 3 bars on an enemy bunny hopping you, but I think theres no drawback to just hugging someone with it and just gouging their HP, maybe a bit more pilot error element needs to be added, as I scored many kills nearly on top of enemies with the Boomstick, but didn't seem to take the damage you would unloading a TOW or HFs into someone under 10M away.  Maybe slight rebalance, lower the damage a tad unless the full spread lands, just getting in as close as possible and hitting them with it all seems a little cheap, but only a little.
EMP: Eh, it's been nerfed pretty hard, and having refused to use it up until now, I have to learn how to make it work for me.  Does feel a bit cheap to sneak up on someone EMP them, then unload a MIRV'd Corsair into their face while they can't do anything, but that's kinda the point.
Blitz: Increased movement speed is nuts, I think it's just a bit too fast, especially when optimization points (when are these getting redone??? I hope soon) for level 25s come into play.  Shooting while boosting, meh, cool mechanic, not really the best part of the ability unless you've got a MIRV ready to shove up their thruster exhaust.  Once again its too early for me to call, but it "feels" like it's a fun ability, and powerful, while it may need balancing, I think it's a good start.
TL;DR:  I think this mech is going to add to the game more than it takes away.  I think once things settle down and there isn't 80%+ players spamming raiders we're going to see the folks that reallly like cqc using this, and flak scouts to punish and harrass the long range chassis like SS and Rkteer, given they can coordinate with their teams and set up good flanks.  I do think some things may need further tweaking and rebalancing, but over all I think we've got a game that has legs again, and this patch has made me WANT to play again, not just log in for an obligatory 5-6 games to kill time.  Thanks ADH, keep it up!