I love the spydercard for certain applications.
First, general impressions. The 6A blade is extremely thin and comes astoundingly sharp. The piece overall has great spyderco quality. The knife has both a straight edge and a sharp point, which sets it aside from most other credit card knives, in a positive way in my experience (for this type of knife, this is exactly the kind of blade I want). Lastly, a note about expectations: the main design critereon was obviously getting the form factor, so ergonomics obviously are sacrificed -- if you judge it based on what it is, you may end up loving it, like I do.
Okay, now about how to carry it. I never carry it in my wallet, I like to keep my wallet light and thin. Instead, I carry the SpyderCard by itself in a back pocket, or in a front pocket. It is extremely comfortable and practically unnoticeable in either place. I've carried it in jeans, dockers, bathing suits w/ pockets, and it's more comfortable to carry than a traditional knife in any of those. Don't get hung up on it being a "wallet knife" -- it is, in fact, a flat knife that IMO is best carried outside the wallet, and excels in carryability when carried that way.
The SpyderCard hasn't replaced my endura or 710 in all situations, but it does often supplement it.
Joe