First a few things on credit scores: They're important, and you should aim to have a good one, but you shouldn't obsess over the numbers. Fluctuations are normal, but keeping tabs on your score will help you make good financial decisions. Watching your score could even help you spot identity theft, because an unexpected score drop could indicate someone is misusing your information.
The downside of these cards as mentioned above is that you can buy merchandise only through the one website or catalog and you must put down a deposit on whatever you purchase. For example, if you were to buy $1000 worth of merchandise you would have to deposit maybe $300 and then finance the remaining $700 on the merchandise card. This will be reported to one or more of the three credit bureaus and will appear as just another credit transaction on your credit report. This will have three important results:
If you have missed payments, get current and stay current: the longer you pay your bills on time after being late, the more your FICO Scores should increase. Older credit problems count for less, so poor credit performance won't haunt you forever. The impact of past credit problems on your FICO Scores fades as time passes and as recent good payment patterns show up on your credit report. And good FICO Scores weigh any credit problems against the positive information that says you're managing your credit well.