I would redirect the smaller (When I say smaller I mean traffic) web sites to your web site, while with the more popular web sites keep them running and have them link to your main web site. Really, I would only buy one or two of them at most and use my money else where. It could backfire on you and be a waste of money.
However, if those sites are cheap and good as well as you have very deep pocket, then it is not so bad an idea to buy them. Considering the very long waiting time for brand new sites to get establish in term of PR and so-called "sandbox", sometimes it could be more worthwhile to buy others out so that you can start things right away.
Conversely, you have to be aware that this is a "short cut" route; though sounds good, it involves MONEY and RISKS.
If the sites are cheap, why not. Search engines aside, they are likely getting some foot traffic - if you redirect their sites to yours, and yours is on the same topic, you will likely get actual traffic as well as their links. Big companies buy buckets of domains to redirect to their main site all the time - not for backlinks, but to take the competition's audience.