I always washed the inserts and the outer shells, but I suppose if you had spare inserts you could re-use the outer. Is it much cheaper to only buy the inserts though? Quite often you see deals on sets but not on the separate bits.
I had bamboo ones and the easy-dry (think that's what it's called!) ones and also bamboo night boosters and found them all great. They fitted my child up to approx 14 months. I'm not a big velcro fan, prefer poppers, but these were good quality velcro so that was OK. I also liked that when I poppered down the rise (for smaller babies) it stayed down when putting the nappy on, unlike a lot of others which just un-popper and leave you with a big nappy. I don't usually like BTP for this reason but again these ones were OK. The outers go a bit pilled because of the cottony fabric but they still look good. All in all, I like them.
For comparison, at that age my other favourite on my son was bambootys (size large as these come up quite small imo.)