Can't comment on the SIMS, but generally I have found most things at Costco to be good quality and they have a no quibble returns policy if there are problems. You do have to be a member of Costco to even enter the store (they check your ID on entry) though.
We got some SIMS from Costco a few years ago now and they are great. I find them really comfortable and I'm not easily pleased when it comes to sleeping at ground level! ;)
You can order online without being a member and their delivery is very good.
Thanks Rainbowfairy, they arrived today and they look great. A quick test in the living room was very comfortable - we'll give them a proper test this weekend
------------- Ex-festival lover trying to get back into camping with an eager 5 year old.
For what its worth I haven't seen them in my local store this year. Seems there is more stuff online in camping gear. Those things that appear on both places are definitely cheaper in store to members.
------------- Christine
2014
April -
BunRoy Fort William 3 sleeps
April / May
Invercaimbe Arisaig 4 sleeps
Bleatarn Brampton tbc July