The Bad:A couple references to non-existant things. Some of the tables refer to ogrun battle cannons, firebox pistols, and scatter rifles; these were either cut or re-named. A couple things are obviously overpowered, for example, and armored greatcoat costs (IIRC) 75 gp, gives a +1 AC bonus that stacks with light armor, and DR 5/bludgeoning. Also, a lot of the IK fluff will be coming out in the World Guide sometime in 2005. Some of that'll be in the Character Primer that came out a while back, though, so if you have that you're OK.
The Good: Everything else
Seriously, I love the IK firearms, but you'll have to download the free preview from the site to get cannons. The prestige classes seem nice, except for the blackclad. It's a druid prestige class that for some reason or another has it own little spells/day and spell list (I think; I haven't looked at it in detail.). The PRCs I remember are Pistoleer, Rifleman, Blackclad, and Warcaster. Warcasters are military mages that control steamjacks on the battlefield.
The core classes are Gunmage, Fell Caller, Arcane Mechanik, and Bodger. Arcane mechaniks are kinda like wizard/experts focused on mechanika (magic/technology fusion), Bodgers are improvisers and battlefield mechaniks, Fell Callers are trollkin combat bard-types, and Gunmages are pistol-combat focused sorcerors +coolness.
The section on Mechanika is very cool, but kinda hard to understand. Basically, you run the risk of permanently losing HP when you create a permanent or charged item with an XP cost of 200 or greater. Mechanika uses some mechanical parts combined with magic to circumvent this; you build various parts of the magic item and then fuse them and add a power source. Because you're making a bunch of little things instead of one big thing, it's not likely that any individual component will have a large XP cost.
I highly recomend it, with the caveat that I'm fairly easy to impress