Well, they don't make an 80 Gig JB3, so it's the only way to get an 80.
For me it was cheaper because I got a killer deal on the HD from a friend with some connections to a computer parts depot.
It does void the warranty (if they know), and if you ever have to send it in you'll want to swap the 20 back in because you may not get the same one back.
The 80 I got was branded as IBM, but I believe it's the same as the Fujitsu MHT2080AT. It has an 8 Mb buffer and I don't know if it's the buffer, my imagination, or something else but I'm pretty sure my battery life has gotten better since switching drives.
NewEgg.com has them for $192 in stock so you do the math on the best JB3 deals you can find.