we have a brita filter - not sure which one, but it's the huge one (holds 5L I think) as dh drinks loads of water and we found continually filling up the small jug a bit boring. It is a rectangular tank-style, which sits on the worktop permanently.
whether you find it worthwhile having one depends on what your water is like - we're in Kent and can massively taste the difference
we have one and have had for about 10 years. We live in quite a hard water area and kettles and things no longer get furred up. I bought a matra one about a year ago (£20) and change the filter every 2 months. Personally from one who drinks alot of water it tastes much better. I also take the filter out every 7-10 days and whack the lot through the dishwasher.