4 and a half years ago when the dealer replaced my stock battery under warranty, I decided to keep a spare handy and bought one of these Yuasa's from Amazon. It ships dry and you add the acid when you need it. Well last month, I felt it was time so I filled it up, charged it, and put it in. Worked like charm, and they are only $70 from Amazon (vs $160? at the dealer). I will be ordering a new spare soon.
Granted it takes a day to prep, but it is nice having it handy if you need it.