I had a Santa Cruz Superlight in my stable . This is a popular conversion for that bike. I opted to go new 29'er -- i had the same opinion as Colin -- didnt seem like a lot in the cost/ benefit ledger and i wanted something that would be a bit more noticeable

I could have spent 5-600 on a set of wheels, possibly the same on a new fork with better clearance, then a little more to replace the worn out chi chi's (cables, chain, cassette) and made it an awesome 650b'er

But then again, i also would have just spent a bunch of cash on an aging platform and would have been miles ahead just replacing the wear items and tires and keeping it rockin'
(if you have parts laying around to do this, or can build wheels yourself with a minimum investment, -- go for it though)