I'm always split on that - go Joel Spolsky and treat yourself right to an Aeron, or go the Paul Graham route and skimp with a cheapo? I've had nothing but Aerons at work for a few years and my butt kinda misses them now that I work from home on a folding chair.
I should clarify where I got "the Paul Graham route" from: somewhere in the Hackers & Painters book, he wrote about how cheap they were in the beginning of Viaweb, with him coding in an old chair that was falling apart.
Sorry I don't have a link, google's giving me no love.
Yeah I've been doing fine with my $25 Costco folding chair - I just threw a soft couch pillow on the seat and try to remember to sit up straight. I think an Aeron could actually be annoying on a hardwood floor like I have at home, with the coaster wheels on the feet sliding all around.
Yep, I have an Aeron at work and an $80 (from Sam's Club) "Office Star Matrex Meshback" or something goofy name like that at home. I use both regularly and have no preference between the two. I would think a startup that needs the meager funding ycombinator deals in would be crazy to blow it on an Aeron.
I've never tried an aeron chair, but with the chair I'm using I'm getting quite a lot of back ache and I can see how a good chair could increase my productivity