I can't speak to the Eneloops praised by others. Maybe they are completely different. My experiences with some four or five generations of rechargeable batteries over the past thirty years has been completely uniform, however, and the message is clear. My experience so far up to and including Energizer rechargeables is that they are worlds better than NiCads but it is still true that
* regardless of how careful you are about charging regime, they only last a couple of years and experience steadily diminishing charge capacity. The "useful life" includes a period of rather short runtime. Either you throw them away when they still have usable life, or you tolerate short runtimes.
* even for a new cell, the runtime for a fully charged battery is much less than that of an ordinary alkaline battery (and maybe a tenth that of a disposable lithium battery). Not good for kids' toys unless you like having them come to you every twenty minutes for a battery swap.
* they self discharge quite noticeably in just a few weeks;
* they deliver lower voltage than an alkaline battery, which may not matter at all or may matter a lot depending on the specific device. The big problem here is that the same devices that tend to eat batteries and thus would be candidates for rechargeables are those with small but powerful electric motors in them, which are exactly the ones that run sluggishly on batteries that deliver lower-than-standard voltages. Does it really matter if a battery-powered electric shaver runs slowly and takes longer to shave? I think so.
Since they only last a couple of years, they are only suitable for a device that consumes batteries quickly enough that a couple of years' use would obviously cost much more in disposable batteries.
Since runtime is much shorter than with disposables, they are only suitable when you are at home, near an always-charging charger, and it is convenient to keep swapping depleted for freshly-charged cells.
On a trip, if you want to get through the trip with a minimum of fuss, use alkalines knowing that you can replace them, at rip-off prices but you can replace them, even in hotel lobby gift shops; or spring for disposable lithiums and you can probably get through the whole trip. For "emergency" use--stick a little battery-powered LED headlamp somewhere in case of a power outage--you definitely do NOT want rechargables.
At the moment, I have only one application I'm using them for: the batteries in my Apple bluetooth wireless mouse.
P.S. The Energizer rechargeables I use in my Apple mouse sit on charge all the time. The charger designed specifically for them keeps them comfy-warm all the time--it doesn't detect full charge and shut off. My Kill-A-Watt gadget is showing 4 watts consumptions... 24/7 365 days a year. This makes me feel very guilty. But if I don't leave them on charge, they will self-discharge to the point of uselessness and won't be ready when I need them.
Annual income twenty pounds, annual expenditure nineteen nineteen and six, result happiness; Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.