They don't supply a full 9V(usually about 8.4) and they die very quickly when they do go.
The ones I have are the NiMH which don't have the fault of not charging up fully if they are charged before full discharge, on the older NiCd this 'memory' eventually destroyed the battery prematurely.
I have 4 x 9V units which I use in the bass, the radio transmitter( 2 in use, 2 back up)and 12 AA rechargebles for my BOSS ME50B as I was sick of tripping up over cables and power pack leads.
They were expensive(?12 UK for the 9V) so work that out in dollars but they are guaranteed to last for over a 1000 charges.
In my view a good investment.