So for my first mech, I randomly unlocked the Catapult K2 which looked really cool with those dual PPC's in the store. The thing is though, this mech had so little armor that it didn't last long in the combat. I could barely scratch the paint off the other mechs, it didn't feel "heavy" at all. I raged, raged and got mad at myself why I didn't get the Atlas and be that "badass" guy that ran into the fights guns blazing, not ran away like I had to with K2.
Anyways after the initial anger period I looked up some info about K2, found builds, got familiar with the mechlab and started experimenting. Apparently K2 is a glasscannon, meant to support the heavies with heavy guns from behind the lines. So started my "experiments". At first I replaced the PPC's with ER PPC's and supplementing it with two medium pulse lazers. Needless to say after firing an alpha my mech barely stayed in a solid form. Other crazy things included 4 large lazers, 2 AC5's with medium lazers and so on.
Nothing worked. K2 still sucked, I was getting manhandled by every mech class, until I got enough points for an XL engine and found place for two gauss cannons. Wow. Even though it's hard to aim these with all the 100+ ping and warping around, they rocked. But I read that "gausspult" was kind of hated online, and I didn't like my two awesome shoulder launchers sitting unused. The solution? Use more guns! I slapped in two PPC's on the shoulders and kept a gauss gun in torso. To make it all lulzy I even fitted a medium lazer in right torso.
Combining it with the "gut feeling" that you develop after a few games(long range shots, taking pot shots, compensating for latency, taking cover behind assaults and looking like a "lesser threat" in combat) this build is the absolute sweet spot for me. I have a burst damage of 40 total which is enough to kill off heavies in a few alphas, I don't overheat at all and I didn't have to sacrifice much armor. I consistently do 500-600 damage in balanced games and even got a 1050 damage in one match, scoring 5 kills.
The only downside is, I'm very bad while being harassed by lights(which is more of a general problem in game right now).
TL;DR: The customization in this game is awesome. With some understanding of what your mech is supposed to do and lots of trial/error, you can create very fun builds. Now I don't want an Atlas at all!