I would suggest going to <a href=”http://www.scmagazineus.com/IDSIPS-2007/GroupTest/50/”>SC Magazine 2007 IDS/IPS test</a> and see if they have any “shootouts” between systems and what they would suggest.
Their buyer’s guide can be found at <a href=”http://www.scmagazineus.com/Intrusion-Prevention-Systems/Directory/66/0/”>SC Magazine IPS Buyer’s Guide</a>
Another excellent source is SANS you can find some archived and recent content on this site. In most cases it is vendor neutral content provided by real folks who use the technology daily. There are also vendor sponsored webcasts but these tend to be more general about the technology than specific about a vendor’s solution. Out of a 30 minute webcast, the vendor may have 10 minutes or less time to do marketing fluff.