Question about building links with Press Releases

So, although I have read a lot and have had some success with building sites up naturally, I wanted to try taking it to another level by subscribing to a press release service.

Pricing and services offered varies quite a bit from service to service.

I was wondering if anyone here could give me some good advice on an effective service to start with? What have you used and which ones should I stay away from?

Thanks in Advance!

Adam