Quite a few of my friends work for Excel. Basically, it is OK as an operation - decent aircraft, usual early starts and late finishes that you get anywhere (but perhaps a bit earlier on the starts than you find elsewhere - a few 0500 local departures!!).
On the downside, the chances of being kept on in the winter are quite low. They have announced they're going long-haul this winter to the Caribbean but they are apparently using this as a means of attracting new F/As for the summer. It's true to say that they are recruiting and true to say that they are starting the Caribbean this winter, but if you think you're being recruited now to fly to the Caribbean this winter, be careful. The two don't necessarily follow.
One of my mates did the numbers the other day and it was something like this for Gatwick:
6 x 737s @ 5 F/As per aircraft x 6 crews per aircraft = 180
4 x 767s @ 8 F/As per aircraft x 6 crews per aircraft = 128
Total crew of 308 for Gatwick - Summer
Winter :
3 x 737s @ 5 F/As per aircraft x 5 crews per aircraft = 75
1 x 767 @ 8 F/As per aircraft x longhaul 7 crews per aircraft = 56
Total crew of 131 for Gatwick - Winter
Basically, you've got 170 crew more in summer than in winter so don't assume you'll get kept on regardless of how good you are.
If you're looking for a start and flying experience, Excel's not a bad place to be and you won't go wrong. If you're looking for a long-term career, keep your options open.