Any battery charger (£15 from local car parts place) will keep your battery topped up - you just need to put in a bit of manual management. An Optimate seems to be marketed on the presumption of "plug in and forget". You can do the same with a standard battery charger at 1/4 the price - you just need to charge the battery (or if it is already DON'T), then undoe the earth wire if it is going to be stood for months on end unused - either that, or start the bike frequently and run it up to the engine warms properly and at sufficient revs the red light goes out for long enough that the juice used to start the thing is replaced by the alternator. - Neighbours will love you for this!
better by far is to just ride the bloody thing. 03 GS, still on original battery. Charged it once this last winter (I think it was this one - the one when we had that 10 day cold snap) and that is it. 6 years and still going strong. I would expect to get 8 years out of a good battery without spending much time charging it.
Boats have a hard time of it. They get a couple of months (if they are lucky) of use, then they get left for the rest of the year. Their starting batteries get totally neglected, and still last on average 4 years with this kind of abuse. Left to go flat until required, rapid charged the day they expect to get used (or started from jump leads/starter pack and charged via alternator)
If looked after, by simply storing fully charged they too last many years (6,7, 8) just by simple charging, store with frost protection (bubble wrap) and then charge gently once in the spring. Job done.
Optimate makers seem to have done a marvellous job of telling people about a problem/need that simply doesn't exist. A side light bulb soldered to two bits of wire will do the same job - attach wires to terminals, allow bulb to glow for a month or so, charge battery when bulb goes dim- that is EXACTLY what an Optimate does, only with a fancy, "digital" side light bulb.