The Heavy Gear campaign is a bit erratic, because the GM's job keeps sending him hither and thither, so we have no sessions for weeks at a time.
We started it in one version of the system (3rd?), then updated the characters to the most recent edition (is that 4th?) about halfway thru the campaign. I own neither. The first version was a variant of the Silhouette system (which I've used in Tribe 8), and the latest version is still d6s... but different in that when things are harder sometimes you tend to lose dice from your pool rather than the target number going up.
When we converted characters I whinged a lot, because you get far fewer skills in the new version AND the difficulty went from 4 to 5. So we all suddenly became crap at things we'd been competent at the week before. The GM eventually gave us a ton of XP to spend to "buy back" all the skills that went missing. And the difficulty seems to be 4 again.
The new edition's aptitudes, quirks, adversities, perks and flaws are nice. Things to hang your roleplaying off, as well as game mechanic effects. Like I'm Confident, so get an extra die once per session but am Uneducated so the target number is one higher for academic stuff.
There's also skill expertise & familiarity which is for extra dice and re-rolls. It's good but confusing to work out if you've got it, because you don't get a familiarity in Stealth, you get an expertise in Survival (which is the category that Stealth belongs to) IF you have more than a certain total of levels in all your Survival based skills. It's quite possible that my familiarity has risen to an expertise when I spent xp but I haven't noticed because I'll have to add up a ton of things to see if I qualify.
There's also an optional "job specialisation/feat" thing. Sorry can't remember the official title. I took Scout (Guerilla).
As is traditional, there isn't enough room on the character sheet for most of what is listed above.
There are Action Points, which are bloody vital if you want to dodge being hit without losing dice from your attack dice pool. Those points refresh every session.
There are Emergency Dice to re-roll your lowest die. No idea when those refresh. We spent ours many sessions ago and haven't had any since. Allegedly you can spend XP on Emergency Dice, but, um, WHY? No way I'm spending my precious, only ever given out at the end of a scenario, xp for a one-time-only re-roll.
The book is huge, unwieldy and impossible to find anything in. Case in point - I'm a sniper, so I wanted to do a called shot to the weak spot on heavily armoured artillery mech. The GM searched and searched and couldn't find the called shot rules. (He didn't have the pdf to hand). Next session... while looking up something else entirely, he found the called shot rules!
BTW the called shot rules are crippling - I had to sacrifice 2 of my 4d6 to do a called shot, which means my chance of hitting plummets (fair enough) but my chance of beating the enemy's dodge also plummets... because roll to hit, then roll to dodge by beating the total of the to-hit. AND if even if they get a crap dodge roll you still have to get through the armour (because apparently aiming for a weak spot doesn't negate that).
So I'm a sniper who only aims once a session, when I invoke all my once-a-session extra dice.
We're mostly having fun despite the system (both 3rd and 4th) rather than because of it.
