My vote is definitely for Iron Kingdoms.
Pros:
+ Really detailed world, with history, religion, politics
+ Interesting non-traditional fantasy races (goblins, ogres, trolls, racist elves)
+ Cyris, Goddess of Machinery. Love her.
+ Arcane Mechaniks and Bodgers. Arcane mechaniks are like Cid in Final Fantasy Vii, grungy, smoking cigars, yelling and using colorful language. Bodgers are more of a throw it together, pray it works version of the above.
+ Guns treated seriously in semi-traditional fantasy. Been waiting ages for that.
+ Oh yeah: battle mechs, though they're about the size of elemental suits in BT. On that note:
+ Steampowered powersuits. Hell. Yes. There's even an "ace pilot" prestige class in the Liber Mechanika.
+ In general, the mechanika rules are highly detailed. If you're the kind of person that loves that sort of thing (like me) then you'll love it.
+ At the moment, supported through PP's No Quarter magazine. Recently its been parts of a magic & religion book they never published (think like SoLA for SR).
Cons:
+ Not as crazy over the top as the mini's game would make you think. The mini game's slogan is, "Play like you got a pair!" The rpgs is, "Scrape Together Money for Bullets!" Kind of a downer.
+ The fans are a little crazy devoted to the above. Also, don't mention DnD 4e ever. If anything, say you're converting it to Savage Worlds.
+ The books are
expensive and hard to get if you're looking to own print copies. The recent pdf announcement eases this some (they're just cover price now) but if you're looking for print, expect to pay upwards of 200$. (We lucked out and walked into a game store that was selling four of the seven books. Add with my gf winning a copy of Five Fingers and an ebay snag last night of Lock and Load, we're only missing two books for a complete collection).
+ Did I mention that the mechanika rules are
really detailed?
+ On that note, if anyone in your group decides to play an arcane mechanik, you need to get Liber Mechanika, the mechanika supplement, which publishes a Arcane Mechanik class that actually works. The one in the character guide is a glorified wizard.
+ The only support is through No Quarter, the Privateer Press magazine, and we're talking maybe three or four pages per issue of rpg content.
+ The metaplot of the books is just around the time that Llael (a minor kingdom) gets invaded, sparking an international war. This war is briefly mentioned, but in order to get details on them, or the big demon dragon running Cryx, or what happened to the northern elves, or basically, anything involving the metaplot, you're going to have to shell out for the mini game codexes. You will not even know who some of the important NPCs are, because they're only detailed in said codexes. It's a little frustrating, especially since you can't find the stupid information online.
That's all I can think of off-hand though! My SO got me into the game when we got together (she loves it so much she costumed as the gunmage on the front cover of the Character Guide) and I've been a supporter all the way til when they announced the project was on indefinite hiatus. In fact, just this Gencon I mentioned to one of the guys working the booth that IK would be a perfect fit for DnD 4e (look, rpg politics aside, the mini game and 4e share common rules) and the look on his face I initially took to mean he violently disliked the idea - and then the announcement of a home system came out just this past week. My gf pointed out that that was probably why I got the look I did