It depends what you're looking for and think you're going to get by fitting one. If you're after more BHP top end, forget it, if your want improved throttle response/rideability then yes you'll see a marked improvement, whether that's worth £125 ish is up to you.
All these things do, is get rid of the environmental/noise restrictions (holes) manufactures put into the engine maps to get through EU/world regulations. Mostly by fooling the ECU into thinking the air temperature is lower than it is for a given rev range, so the ECU puts is a bit more fuel, you can do the same with a simple resistor, that cost pennies. Don't confuse these with a power commander, the latter is a proper engine mapping system.
If you've got an aftermarket end can/exhaust and/or you've removed the O2 sensor it wont work as it should.
