I use rechargeable where I can and prefer them to the standard ones. A couple of thoughts as far as i know though.

While they are lower voltage, they can have a much higher capacity than alkalines and keep a device running longer. Not all devices are suited to them though.

I believe the rechargeable generally have a relatively high self discharge rate. i've not used them but the enloops BillD mentions are supposed to overcome this and i'd guess could be the best option if you have the batteries sitting idle for long periods.