I rode with a speedhub for over two years and like(d) them alot.
Sold my Nicolai with the 'hoff on thinking that I would have to give up riding (but thats another thread).
So I've recently bought another bike (currently being built up), and I've got conventional gears.
I think the 'hoff works well on a hardtail, and particualrly a frame which is designed for it (Endorfin Spped R, which I had, and wish I still had).
Although I had the 'hoff on the Nicolai (specifically designed for the speedhub) , I'm still not 100% sure (which doesn't mean I don't like it).
It just seems that the hardtail seems to suit it much better. No chain tensioner and the weight seems to be coped with better on the lighter hardtail, than a heavier full suss.
Drag is there, depends on how much climbing is involved where you ride.
Expensive – yes
Excellent means of changing gear once you get used to it.
Probably will buy another (not just yet though) on a hardtail frame though.