Mike,
Our company uses Vocus( which owns PRweb) about 2-3 times per week. In the past we have used other PR release services but have found this has been the best. Vocus is a annual paid service ( a bit pricey 5K) but considering it allows you to send out 1 press release a day which I would not recommend, the largest benefit is that the service allows you to send your press release directly to new editors emails, magazine etc. This service has gotten out products in Southern Living and a few other magazines- which is priceless.
Make sure you goal is to create awareness rather than quality backlinks- make sure your press release are something people actually want to know about- then the natural process of high quality backlinks will come.
Good luck.
Chad