I have the 6500 brifters, (got em for $10 cause they were toast) but now I've overhauled em they're sweet.

Shimano's nasty grease is your biggest problem with STIs, but also with these 9spd ones they can start to jam when you try to shift to smaller. I had to soften the flipper's spring and stiffen the big lever's spring on both of my levers. Took a few goes to get enough difference in tension. Also, these are a fair bit trickier to reassemble than the 6400s, which although far uglier are a better thing because they don't seem to suffer the jamming problem and based on overhauling a couple of sets I'd say they'll go for decades with proper grease in them. They're also more accurate, pulling more cable per shift.

Although a 9spd drivetrain does shift a lot more sweetly... you need good cable housings for it though, with Shimano's geometry.

Also, you can have pretty much any Shimano derailleur you like (late 80s DA and Dynasis excepted, along with pre-indexed), as long as the chain fits in the RD cage (you can just shim narrow pulleys); on one bike I have the 6500s shifting a 6400 RD, and on another I have 6400s shifting a Deore with a road cage.