Yes, it's absolutely worthwhile to have Windows 7 bootcamped. I've been running W7 Ult 64 on my Late 2009 17" MBP for over a year now and absolutely adore having it as an option for gaming.
For awhile I was having some crash issues on the W7 side, but a patch fixed that. I believe the BSoD I was getting was related to a bad driver for the graphics card, but as I said, that issue has been resolved now. I haven't bluescreened for a very long time now.
As for space I went with 50GB on my bootcamp partition, but now I realize I would have preferred 100GB. I run Adobe Creative Suite on there ('cause all my other machines are W7, and I'm too cheap to send Adobe double the money just to use it on OSX). CS uses a lot of scratch disk room for what I do, and I prefer to keep that confined to my internal hard drive. I'm constantly having space issues with just 50GB.
For gaming I keep an external HD with all my games on it to conserve space on the internal and that works perfectly. I've run SWG, EQ2, AoC, STO, DCUO, Non-OSX Steam games, and others on the W7 side from my external, and they all run great. I do wish there were OSX clients for them though.... I play my games on OSX whenever I'm able--EVE Online, CoH/V, WAR, SC I & II, Portal 1 & 2, Minecraft, Fallen Earth.