I find them easier to maintain than other forms. Initial training was simple enough with most, although the gala didn't pay nicely at first. I summer prune to retain shape and stop them getting too 'hairy'. I have trees which span 3ft up to 8ft, most around 8ft tall with 5 tiers, the less arm length the more pruning and less fruit.
Tip bearing tress are obviously out, so that's most of your early apples, but anything else will do although remember they carry less fruit than an open centre or bush. If you're planning on cookers you'll need a reasonable width to produce enough to make it worthwhile.
Blossom appears to be a bit more susceptible to late frosts and the nicely presented fruit makes easy picking for birds and squirrels. Kidds Orange Red has made a particularly attractive and productive tree, but you're best to pick something suitable to your conditions and then shape it,just don't be put off by the initial chop.