Martin - the battery tenders work very well - I have an optimate 3 myself, but I've found that the bike should be on its centrestand, and you should check the battery water every so often - if not, the water can bubble away and wreck the battery, and the charger. Centrestand because batteries should be level when charged....
Just my 2c

 

FYI - I check water about 4 times / year - top up but always in limits.

From my experience - Battery tender reduces chances of R/R problems by not putting "strain" on charging system due "weak / discharged" battery.