I have used ian.com (TravelNow.com) for [almost] 5 years. I find them to be OK. The original TravelNow was bought out shortly after I joined them, and by fall of 2000 there were customer service problems. I picked up another company, Lodging.com and have been very happy with them. Good products, good commissions and great service. I also have used World Choice Travel (wctravel.com), which is a good company, owned by Travelocity, and is geared toward the individual site owner. They have content for your site and also have a turn-key travel page that can be installed on any site. I am also beginning to work with IHSadvantage.com - So far I am impressed, they recruited me. (ex: InternetHoelSolutions.com)
IAN.com is now my fourth-in-line in sales and in commissions.
Just my 2-cents worth - hope it helps a little. Go to the above URLs and check them out. I may be able to answer more specific questions by PM. If you have a "track record" in affiliate marketing, I may be able to introduce you to the "right" folks.
G/L