The value of PRWeb et al is not in the direct links you get from the release - though they don't hurt. The value is in the networking/lead generation that develops from someone actually reading the press release. A well crafted and news worthy press release still has value for making new connections. The issue is the signal to noise level on those sites.
Too many press releases without much for substantial content == a lot of noise and of little use. If your business is not a player in the market space you inhabit, then you're wasting your money because very few will pay attention intentionally. You'd have better results spending your money on Adwords. If you are a player in your market, then it's a pretty safe bet that someone is watching you and your press releases will be newsworthy providing it's a newsworthy item.
If you're strictly looking for a sale - I'd suggest you spend you dollars elsewhere.