paid links are always beneficial till they are relevant but as long as you don't get caught by search engines.
All links are beneficial, because a link is a link! DoFollow links that are related to your niche are even better. When you are paying for links, you are better off buying links that are very close to your niche.
If they are premium links and are well hidden,these things can really sky rocket your site up high in the SERPS
Paid links will not help your search engine ranking. The search engines, especially Google, have become very proactive against them. Google has created a filter that detects who is selling backlinks, and no longer considers those sites quality ones. If you buy from them, Google will deem your site low-quality, too.
The only way that you can buy links is to buy from sites that are approved by Google. The links cannot be used to boost your search engine ranking. In fact, Google requires any paid links to be nofollow.
If you're purchasing paid links you can do so for several purposes as well.
If you're looking for traffic and purchase a paid link on a site with teh same niche as yours you could get some trickle of traffic.
Paying links for backlinks is a little greyhat for google and it might cost too much to get substantial amount of backlinks.
I could go on for hours however essentially it boils down to how you use it. What you want to achieve and what's your budget.
There are also other free wasy to receiving traffic and backlinks one of my favourite is through facebook for traffic and forums for backlinks. Just my preference though because both it enjoyable for me