I've actually used these twice when I needed small loans. I think the 1% fee is worth it for smaller amounts. Both times I did use them they were less than $5000 so I paid less than $50 to carry that loan for about a year.
Of course, if you don't pay it off in time, it's a horrible deal. I just wrote the check to myself once and the other direct deposited the funds into my bank account. It was really convenient, and I probably paid less for that than I would have had I taken out a loan with a bank.
Barclays even has dedicated page when you log on to your account to see when you took the offer which makes it easier to track and thus not forget when to pay it off.
Just my experience with it, but I've definitely used it and will more than likely use it again in the future. The amount taken out was less than 30% of my available credit limit with them so it didn't hurt my credit score, but that is something to consider.