I have the UK ones - I like the bright colours of the US boxsets, but I found handling them a pain. I did get a couple of the region 2 plastic boxsets, but they were so expensive that I decided to wait for all the slimpaks. They were cool though; I especially liked the capsule-like Enterprise ones.
The only ones I still have in plastic are TOS and TAS (the latter hasn't been brought out in slimpak so far, don't know if it ever will be).
I do like the slimpaks, though, they look great together on the shelf - once again, the nicest set is probably Enterprise. You can't really tell here but they have a sort of glossy hologram effect and look really nice:
Here's all of VOY - I do like the ease of getting to the discs in their individual cases as opposed to the plastic boxsets.
Those cover printouts above are wonderful, I adore the Captains set.