Have a look at the product test on the Auto Express website:
http://www.autoexpress.co.uk/product_te ... aners.html
They don't seem too impressed with the cigarette lighter jobs, and in fact they recommend paying a couple of quid more on one of the cheaper rechargables.
I use a Halfords one which I think was recommended in a similar test when I bought it a few years ago. It's OK, but I think it's best used very regularly with the occassional hoover with a proper vacuum cleaner to mop up anything it's missed.
I doubt if there's that much to choose between the cheap 12v ones at around £15-20, but for best results I always clean the filter on mine after each use as it clogs up very easily and this badly affects the suction.
Another gripe with mine is that it's not good at getting into hard to get at bits (mainly between the door sills and the seats) - that requires a proper vacuum cleaner - but this might be something to look out for when buying.
Another pain is that on mine you have to hold the switch on the 'on' position all the time, which is a nuisance. I don't know if they're all like that, but presumably it's to stop you perhaps leaving it running and forgetting about it and flattening the battery. But to get the best suction the engine has to be running anyway.