There's been a few receont posts about Great Crossbows, and I've shared some comments about my experiance and opinions there.
But I thought I'd add a few build ideas. (well one actually)
This is not a bad weapon....and Uber ones can be gotten very cheaply.
But as the owner of a one-trick pony, I would suggest not centering a build around this weapon, or any one weapon. But make a more generic build that could also use Great Crossbows.
I suggest a Rgr.
Probably Human for the extra feat, but that is the only real reason for suggesting Human.
You need three feats to make a great Great Crossbow user.
Exotic Wep Great Crossbow (of course)
Rapid Reload
Improved Crit Ranged (which IMO any Rgr should take eventually anyway)
Rgrs get all the other important feats you might want.
Plus Rangers are defiantely not one-trick ponies.
You could then use whichever ranged wep you have in a given situation... Bow, Crossbow, Great crossbow. And be good at all of them. Plus TWF (prob not Tempest though )....and stil be a very good char IMO.
Splashing Ftr 1 or 2, could prob fit Tempest into a build as well.
Or for the really ranged enthusiest, one 0r both repeater proficencies.
Note, non-proficiency penalty is only -4 to hit. The same as Manyshot!
So you don't really need to be proficent to use them.