I use the site off and on for many of my AC bookings.
Positives:
- Tracks to your corporate spend for discount purposes
- Sometimes offers flight routings not offered on AC.com (I believe it is the same engine as on the i-phone app).
- Discount on Flex and above fares
- Integrates flight pass, so you can compare cost of using your flight pass versus the cost of the different fare buckets
- Retains credit card info
Negatives- Sometimes comes up with absurdly high pricing. in those cases I will compare with AC.com and then call in to book manually and get corporate rewards discount (if it's worth the bother)
- retains credit card info (if you are worried about the site being hacked)
- Since it is a different site, you need to rmeember whether you booked on this site or AC.com to find your reservation in case you wish to modify