I'm thinking of getting one, it's interesting to see people's opinions of them.
For me it's more about having an extra tool available to me. Most of my mileage is commuting, and whilst I try to get out for a couple of longer rides each month, Mrs C does like me being home of an evening.
This is a good way for me to get an hour of solid exercise and not be out for too long.
Besides anything at the moment I've a separated shoulder so can't ride a bike at all, driving me mad. Likely to get one this week just to get back on a bike at all.