I know that I'm late on this thread but I did a lot of research before converting to an IGH, and this is the type of thread a relied on.
I've done just over 1,000 miles in the last 3 months on a SA 5 gear hub which I put on an old (early 1980s) steel frame Raleigh.
The bike weighed less after the changeover namely because I changed the rear wheel to an aluminium 700c tourer and took off all the other bits and pieces. It took about a week to tweek the gears as the tension stretched the gear cable but after that I haven't touched it.
The ride is different, although you have less choice it makes you ride within a smaller range of gears, I've even found that I now change up on climbs that previously I would have sat in a lower gear.
With regards resistance, I've done the maths and read all the articles on relative gear efficiency and the facts are that there is a slight deficit compared to well maintained and top end derailleurs, but, once you're pushing past 300-400 watts, that relative resistance reduces down to negligible.
I much prefer the ability to change down at a stop so that I can pull away more easily and it's not too much of a hassle to take the pace off to change on the go. The best bit is that when I change gear, I know it and there's no messing around with the chainline.
I was nervous about changeover but wouldn't change it for the world. For the hacks that moan about weight and resistance, it's easier to lose a few pounds than spend thousands on carbon fibre and if you're pushing at less than 200 watts then you're not a serious cyclist.
It was on the expensive side to do the conversion but I've learned a lot in the process and have finally researched ride position and optimal cadence, so by the time I've got round to changing my crankset it'll be as good as it gets.
I had thought about making a fixed gear, but after speaking to a few people and considering knee joint mechanics (I'm a chiropractor, so I know about knees) a single speed would shred my patellae within a decade.
So, this is my last bike (I've had about 20), I use it to commute, I'm going to start racing with it this summer (105 and 127km long races respectively) and I think I might do a tour or two as well. I don't think I'll go back to derailleurs because for the expense of perhaps replacing the chain or hub in the next 40,000 miles or so is nothing compared to running a car doing the same mileage.

Hope this helps.