I have tried the Draper one that can be bought in Halfords for around £25. I think it would be fine on a normal battery, but i have killed 2 on my Optima battery, despite it saying it would be ok.
I have also got a solar charger that i picked up from www12voltz.com . It has an output peak of 125mA at 15volts operational voltage
7.2 - 9 amps and 0.5 - 0.625 amp hours a day based on 4- 5 hours of average daily peak sunlight.
dimensions 12" x 4" x 1" (Sits between the rear speakers nicely) It was about £28 delivered.
This unit works OK, but didnt solve the issue i had. I think I have a drain in my system other than the power being used by the tracker / alarm.
I have heard good things about the AccuMate trickle charger, if you go for a wall mount system (provided you have power) I persoanlly, am using a £45 Halfords own brand battery charger/maintainer. Works a treat, and is also a really decent battery charger, so you get two great devices in one.