its worth it, the other image editor progrmas out there are nothig compared to it. the next best thing would be paint shop pro - thats £80 but nowhere near as good.
paint shop pro can do quite a few things, its good enough for starters, but if you want total control and a powerfull editor with tons of effects and tools then photoshop is the way to go.
but then i alway its down to the user and not the program. you could have photoshop and be rubbish at image editing or have paint shop pro and be fantastic. it all depends on the individual.