Nimh's are great for most applications I've found. However, they have a very high self discharge rate. They loose about
.5-1% of capacity a day! Things like smoke alarms, clocks and other things that have a very low power draw I've traditionally used alkaline batteries for. However, I recently tried 'precharged' or
low self discharge nimh cells in a few things and have been very happy so far. They're a specially designed nimh cell with a low self discharge rate. Its still higher than alkaline, but IMO quite acceptable. They only self discharge 15-30% per year (30% = .12% per day to compare).