JaccoW

So far I really like the 90mm ones on my SilverAce, the front hasn't been bedded in yet but the rear offers enough stopping power on its own to make you shoot off the saddle if you're only holding on with one hand. The lever feel is different from other types of brakes, if you're used to the instant grab of a rim brake or discs these might feel mushy at first. They have a self-actuating action where the brake power increases a lot more when you increase the pressure on the lever. It's different from the more linear feel that canti's offer for example.
All in all they make it easier to bleed a bit of speed for me but can still bring the bike to an impressive screeching halt. Non-conpressible cable housing helps though.

I haven't ridden the 70mm version yet but from what I've read they are perfectly adequate brakes. Not so great in the mountains but on flatlands and rolling hills they work just fine and are virtually trouble free for decades.

One thing you see manufacturers like Pashley do is a 90mm in the front and a 70mm in the rear. I think it makes a lot of sense for a lot of people.

As for the 5-speed hub, I have the older XL-RD5(w) model which shares a lot of parts with the old Sturmey Archer AW/AB hub and that one can be a bit finicky to adjust. I think I've gotten the hang of it by now but the newer RXL-RD5 should be more forgiving.