So your trying to say the alfine weighs 5-6lb?

cough* Who's the sad git that weighed his Alfine hub before building it up into a wheel to go on his bike *cough...
ME!
Mine was 1668g unadorned. Which is what, just over 3.5lb iirc...
Weight difference between a derrailleur system and Alfine setup... Hmmm... Well, if you're running full XTR I think you'd probably gain 2lb or more, but if it's Deore or SLX, or even XT kit you're running, you're only going to gain 1 to 1.5lb overall.
Noticable? In a dead lift of the bike yes... When out riding? Really, it is only very slightly noticable that all that weight is in one place. Certainly my bike doesn't feel that rear heavy that it's unbalanced when its wheels leave the ground or anything.
Gear ratio wise, I was running 32x20 on my 29er, but have just bought a 22T Nexus sprocket and had a 34T chainring lying about so they've both gone on. This will only lower the gearing very slightly, approx 3% overall, but I'm hoping it will suit me. The 32x20 on a 29er at least (10% larger rolling radius on the wheels) was just slightly too tall for me... I may even end up dropping a tooth on the front perhaps and fitting a 33T ring, but the 34T is what I had lying about and the 32T ring that came off was way past its throw away date...
If you want an Alfine gearing calculator Spreadsheet that I knocked up (this applies to anyone), then email me (email is in my profile)... It's all singing, all dancing etc, and will tell you all about your overall ratios, gain ratios, gear inches, mph etc in all of the gears and for any wheel size and gearing combination.
And performance wise, well only done maybe a dozen rides on mine so far, so not quite as well placed to comment as some, but not had a single issue yet. Yes, it's a bit slower under shifting to an easier gear than a derrailleur setup (you have to back off slightly), but otherwise it's great. With my current gearing my 1st gear is the equivalent to a 22x27 derrailleur setup, and my top gear is equivalent to 32x12. So I can climb most hills without getting off and walking, and I only spin out above 20mph on the road in top.
Personally, I can't recommend them highly enough. I know a few people with them now, and everybody raves about them. It's pretty much a case of singlespeed simplicity, but with the added bonus of 8 gears rather than one. Sure, it's a bit heavier, but if you're not racing will that matter? In fact, speaking of racing, there's a guy in the US that posts on MTBR forum a lot that to prove a point, took his Alfined 29er out on one XC race instead of his normal race bike. He won funnily enough!
Oh, and the Alfine is/was a godsend through the snow and the mud. Haven't washed my bike once since I got it (about 2 months ago), gave it a mild brush down yesterday just to get the worst of the crap off prior to fitting different sprockets and a new chain. Everything just works! I was getting fed up of spending nearly as much time cleaning the drivetrains on my other bikes as I was riding them through the mud, I gave the Alfine a go. Now I doubt any of my other bikes will get a look in until the ground firms up somewhat!