Obviously a link is a link and google is not going to track down that you paid for it unless it is in some way glaringly obvious. Just get some reviews and look into the reputation of who you are buying from and don't just go for the cheapest deal.
In my openion it is not a good idea to purchase links as till the time you pay for those links that will be beneficial for you but as you stop paying to these sites they will remove your link and then you will be no where. so it is always good to produce quality back links by using backlinks activities like articles submissions, blog submissions, directories, social bookmarking, squidoo lenses, hub pages, facebook, press releases, classifieds, forums and others.
Done right, it's an excellent technique. Keeps links relevant to your niche though, and don't overdo if you want to stay under Google's radar.
Buying high PR links will help your site ranking. But if you got caught, you will get penalize from Google. So I think its not worth buying links. Instead, you should find ways to get links from quality sites. Its risk free.
It doesn't hurt your site as long as they aren't reciprocal links, although most sites require to pay monthly to have your link placed on their sites. I personally prefer permanent links.
Don't be obvious when buying links. Never buy from sites that would disclose that they are selling links.