Personally, the less crap I have on me, the better. I know some MTB'ers like to dress like they're going to battle... but for my local trails, there really is no need for riot gear. In fact, I pretty much roadie-it-up in terms of riding apparel: lycra. I will wear baggies in the cold winter or for more DH oriented trails, but the less material, for me, the better.
I've been wearing a hydration pack since I got my dropper seat post on my Jekyll, but always kinda hated it. It's nice to not have to reach down to grab a bottle, but sucks because of the extra weight on my back. However, I've needed a way to carry some small tools, so I wore it.
During the last few rides, I've been trying to find ways to secure my tools without a pack. Since you can't really use a seat bag on a dropper post, I tried to make something out of a toe clip strap, throw stuff in my jersey pocket, etc. and it all failed miserably. I considered an Awesome Strap, but wanted to check my options at home before spending more money. Yesterday, however, I remembered Steph's words of wisdom.
My wife bought me the Timbuk2 Rump Roast hip pack earlier this year for BMX - but I haven't been wearing it. For the type of street riding I do, it was just too bulky for BMX (WeThePeople is coming out with THIS seat/seatpost combo - the Smuggle Seat - which I am going to buy). However, for MTB - when you don't have a seat bag option - it works fantastic. I'll I really need is a spare tube, tire levers, C02
I took it on my local route yesterday and it proved fantastic. I had to re-adjust the belt a couple of times, but aside from that, it held on securely. The best thing was I didn't have 100ml of water on my back, and Cannondale did a great thing by adding water bottle mounts to the Jekyll.
The Timbuk2 bag is a great idea. I can imagine using it for longer CX excursions, or rides with the wife when I need to carry a little more. All-in-all it looks like I found a new purpose for that little bag. #timbuk2rumproast