I am not much of an authority on them but I did look into them briefly..I just never found a convincing reason to pay a monthly charge of $15-$20/month to do what USPS.com or my local Post Office did. It did seem to be a better value proposition if you are doing lots of mass mailings or other database oriented mailing tasks (small business, regular mailing list activity). Most of my mailings were unique purchasers and Ebay (a significant portion of my mailing) does offer discounted postage purchasing on site.

TL;DR Not sure what value i would get from the extra $15 bucks per month.