I've had that Halfords one for a few years and it is OK but not really ideal for big engines or hard use with say a big flat battery in very cold conditions.
I have also have had this powervamp one for a year
here
and it is very good indeed. This winter it has started a piling rig, a dumper and today a big loader thing. It has also happily started a 8.1 v8 boat in the midst of winter.
I think it has the same battery as the powervamp one in my caterham and can be changed if and when it dies.
I think I paid 150 for it it but it may have been a special show price at autosport.
Will