DEFinitely a necessary item. I got an REI version on sale some months back. Boy was I lucky! It holds 3 liters + room for some spare gear. (I always need to carry some ostomy supplies and a spare pair of shorts, just in case. Also cool to have a place to stuff the fleece jacket once I'm warmed up.) It's WSD and streamlined, so fits really neat and snug. AND it was on sale. I even got one more for UK Elephant. But, it's a rather pale and boring shade of green.

So today, REI sale catalog in hand, I REALLY splurged! Got one necessary item (bright yellow bike jacket with removable sleeves) and lots of other "stuff" -- Leatherman tool for DH, Topeak Alien II for UK Elephant, new and bright red lightweight 35-liter backpack (equiped to take hydration pack, but way too big for doing rides), and and and ... all in all spent over $300. Even without the "caffeinate me" bike socks I was thinking of getting for Trek420 (Sorry Trek, they only had men's large left). This is what happens when you get taken in by what the signs say you're "saving".