They have a torque limit, which means you may not get a real granny gear. I'm running 44 x 21 on a Shimano Nexus 8-speed (Mrs Z's bike), which gives a 20"-something bottom gear and it's coping, but I wouldn't go any lower, especially if you pedal like Chris Hoy.
The Shimano ones need regreasing, or at least "dipping" every 2 years as preventive maintenance. Neglect this, and by the time it's started to play up, it's usually knackered. Few bike shops really understand them, and they're uneconomic to fully strip and fix, even if spare parts were available.
If these don't worry you, go for it. The Shimano one is quite nice to use.