I have an Oxford Oximiser 900 for a couple of years and still works fine. I like the fact it has an LCD display so I can see the state of charge, plus they are 1/2 the price of a Ctek or optimate. My bike came with an optimate cable and you can get an adaptor to work with the Optimate connector. I also have a Ctek MXS 7.0 that I bought for the leisure battery on my van, but that is a bit heavy duty for a bike and was pricey.
If you get a cheap one and you have an AGM battery, make sure it will charge an AGM battery. Some, like Ctek have an AGM setting.