[Wobs]

From what I've read up on them:
Evacuated tubes are cheaper, and more efficient, but don't last as long.

The flat plate design is more expensive, but longer lasting.

Swings and roundabouts so far.

I'd love one, but don't use enough hot water to warrent it. And in winter, its doubtful it would provide enough for a heavy user without boiler backup.

The nature of the panels means that a south facing roof would be ideal, which limits them for many houses in the uk, although of course there's degrees of how appropriate a house's orientation is.

In the uk, there has been studies which show how little you get in winter, so you just need to temper you expectations.

The costs I've seen have been in the low 1000s, but vary. Payback would be rather long for most in the UK, but some may wish to take the plunge regardless (someone has near us).

The other alternative is making one yourself, although it won't be nearly as efficient, you could paint a radiator black, etc. Ultra cheap, and as such may be worthwhile for some.