I say try them for yourself. I'm speaking especially in regards to Pligg.
Templates can be modded and exploitable code cleaned up. Pligg is an easy install and just as easy to mod - even with limited PHP skills.
Do the market research for yourself. Free is good when you read the manual and take care to interact with others running the software.
A new Pligg version just came out as well -
www.pligg.com