Never personally used them but a buddy of mine had a really good experience with them. If these will be the first critters in the tank, I'd order a small pack first even if you have a large tank, just to make sure the tank is well suited for life.
All my cleanup crews for the past 10 years have come from John at Reefcleaners.org. Never had an issue with quality or price. Also, he keeps his tanks fish free so no worries of ick/velvet/brooks sneaking in on a snail shell.
I would recommend going small on the CUC rather than large. If the CUC has nothing to munch on most will die causing your nutrients to go up. Plus, it's just wasteful. Personally I went with a "30gal" CUC to start on my 220. As time goes on I'll add more. Just be careful about adding a bunch of snails and crabs that have nothing to eat. The crabs will certainly eat the snails then.
+1 to the above. I always order from reefcleaners too.