I have 23 cardinals in a 38-gallon planted setup. About half are wild-caught (which I think is more commonly what you get at the LFS) and half are tank-raised. I bought in three batches. The first wild batch from the LFS, I lost none. The second wild batch from the LFS was carrying ich, and I lost about a dozen (some from the first, some from the second) before things settled. (I know, I know, should have QT'd.) The third tank-raised batch from msjinkzd, I didn't lose a single one. They are all very colorful, happy, and active in my soft-water setup (pH around 6.5, KH around 1, GH around 6) at 77 degrees.
My experience and what I've read is that they can be sensitive at first, but once they're stable and in a stable tank, they are quite hardy. I have redone the substrate and otherwise mucked around in my tank (re-scaping etc.) and haven't lost any since the ich incident.
tl;dr: great looking fish, hardy once acclimated, highly recommend.
Bump:
I would say cardinals are more difficult than neons, but they are still a pretty simple fish. I read somewhere that most neons are tank bred where as cardinals tend to be more wild caught. I'm not sure if that is true or not, but it kinda makes since and explains the different in difficulty.
The best tips I can give you is to find a reputable source. Buy a few more than you actually want, because it is quite possible that a few will die from the stress of getting moved to a new tank and finally make sure that if you buy smaller/younger fish, that you allow them time to grow out before putting them with other fish. You can typically find them pretty small and other fish will eat them pretty quickly.
Agreed on all of this. Most you find will be wild-caught. msjinkzd often has them tank-raised, and mine from her have been very hardy (no losses yet, and it's been months). Also, I'd guess that tank-raised fishes will do better in your higher pH setup.