I haven't purchased the pre-built packs...i buy the type of snail i need for the particular job I need fixed.
I like their Astrea snails... good eaters and hardy. But, they can't flip themselves over so you have to watch if they tumble off a rock. The only ones I've had to flip have been ones i knocked over though.
Their cerith snails are good... the little ones get into rock crevices very well.
Nerite are good as well, but they have been known to leave the tank...i haven't had any do this, though. They will sit up at the edge of the water line sometimes.
Nassarious eat only meat... good to clean up after getting the fish.
I wish they did trochus snails though...i bought 5 of those from my lfs and they're doing great.
They usually send more than you ordered to account for any doa. Which is very nice of them.
Depends how much algae you have for them to eat to determine how many to purchase, though. Too many and they starve and can foul your water... but the nassarious snails will love the extra meals and fix that problem for you, lol.
I have a 75g with around 5 trochus, 6-7 astraea, ~10 nassarious, ~6 nerite, and who knows how many dwarf ceriths! My rocks are clean of algae... but my back wall is covered in GHA (on purpose attempting to out-compete a new dino bloom due to low nutrients). The snails attack the back glass some, but mainly focus on the rocks.