I use stamps. It is worthwhile if you are doing large buys as often as me. Otherwise I would say no. While you do get slightly better rates than going to post office and the ability to ship more than priority unlike usps site, the monthly fee could easily negate any savings unless you are sending in good volume. It is also easier to use than paypal system by a good deal, and I love that is keeps the customs form filled in for entire sessions so I only need to enter things 1 time and only change the addresses. That is a massive time saver.
If you are mostly going to use small flat rate I would just use the usps site.