Nothing wrong the Odyssey batteries, the last one i had lasted over 11yrs (accroding to the build tag)...same as any other high quality battery...they just don't like been left for long periods without a charge...again the same as most batteries.
I replaced my Odyssey with another Odyssey (although i went for the PC12xx)
Be careful with placing a normal (wet cell) battery in the boot. For one its illigal without ventilationa and 2, if it leaks it could cause a fire = no more gtr.
as terry, Ant and others stated, trickel charge is the way to go.