They are much better with ISOFIX base mountings otherwise they do tend to rattle and slide around, even with the belt done up if they're not sat in it.
If you're looking to buy a seat I highly recommend the Diono Monterey 2. Ours must be nearly 6 years old now and my 10 year old still loves it. I went with the intention of buying an expensive Recaro seat but she sat in a few and chose it. Loads of height and width adjustment on it, and the head support means she sleeps a lot. The only disadvantage is the base is quite wide so if you only have a small car it might be a tight fit.