It's always good to see other Steam-crazy people on the boards.
The main issue with the class is making it useful in dungeon-crawling, since that's a fairly core element of most campaigns. This is where I find alot of other examples fail, as they focus on item creation which usually requires a lab and time.
Right now the Engine Creation ability in your class looks blank (unless something's screwed up in my viewer), so it's hard for me to evaluate whether it would be useful. Without something that gives you power and/or flexibility in combat (beyond what items you have) the class will be underpowered in my opinion. You'd end having a Wizard's combat stats with a truncated Rogue's skill-list... which sure wouldn't appeal to any of my players.
Most of the other abilities, and specifically Create Pressure Build could be useful in a campaign which centred around having constructs do your fighting (your Engineer would take on a Cleric's role in this case), but would otherwise be mostly for flavour.
As for the prestige classes, the balance of these usually doesn't matter so much as it's very campaign specific. In terms of flavour, it looks pretty cool.
Note:
There's been several attempts at a useful Steampunk core class in books and on boards over time. I'll pimp an early attempt on my part
here. I've changed some of the mechanics since then, but its stylistically the same. We've been using it in my campaign and it's worked okay so far. Maybe it'll help give you ideas.
Other attempts on this style of class (other than the stuff you obviously have) are the artificer from
Sorcery and Steam and the gnomish artificer from
Magic of Faerun. I didn't like the way either one works, but if you're looking for other people's attempt on the class, those are good things to flip through.
