Generally I don't like them as the nap is only about 5 mil and they don't carry enough paint, you tend to spend more time loading them up than actually painting. The one area where they are come into their own is if you are painting the risers on a stairs......risers painted white/treads stained. Some pads have little wheels on them and you can just run them along the tread and paint the riser very accurately.
If you are doing a lot of painting and want a brush that will last a lifetime and 'cuts well' get a Purdy paintbrush with flagged tips made from nylon. MRCB sell them between 20-40 euro. Best brush in the world.
If you don't like the 'orange peel' effect that rollers leave on walls use a big 10'' wall brush to brush them out. You will have arms like Popeye afterwards!