I obviously don't use them, but for a spur of the moment weekend getaway they're pretty good (as GNRMatt says, there are normally some duds in there).

For European travel, you get longer stay, but again booking is time limited to spur of the moment so it might be hard to fit into your schedule.

If you start looking well in advance of planned travel dates and have some flexibility on who you fly you may be able to find fares as low at the E-savers. The one's I looked at earlier this evening for PHL-MAN were in the $600 range plus taxes/fees R/T so weren't a super bargain - probably about $750 all in. For example, DL offers a one-connection each way ticket on the same days as the E-saver for $920 all in. If you want to stick to the *A and don't mind the drive to EWR, CO has non-stop EWR-MAN for $834 all in.

Jim