The cheaper ones are pretty good. Fresherpack's Eiffel is very good because it has the moisture trap, I can't recommend it enough.
However it is a fair bit more expensive than the one Ed has linked to. The Andrew James ones are supposed to be pretty good but what you'll find is you get what you pay for, a £40 vac sealer will likely not last as long as an Eiffel but will probably do the job you want it to.
I've had cheaper ones before and they've been fine.