As good as Alkaline and rechargeable!
Most of your nifty gadgets take batteries. Good quality alkaline disposable (primary) batteries such as Duracell or Energizer are very expensive, ranging from sixteen dollars for twenty four AA batteries if you buy them at Costco, up to over two dollars each if you buy one or two at a convenience store. They are also now considered hazardous waste and are illegal to put in the trash!
Until recently however, your only alternative was to use rechargeable Ni-Cad batteries. If you have ever used the Ni-Cad equivalents of the common battery sizes ( AAA, AA, C and D) you know what a disappointment they are. Not only are their capacity, at no more than 600 mAh for a AA, less than a quarter that of a regular Alkaline battery, but their internal resistance is so high that even when freshly charged a flashlight using Ni-Cad replacements is dim. Add to this the fact that they are finicky. You must fully discharge each battery before fully recharging them. Failure to follow this strict regimen will result in a "memory" effect where the battery will no longer be able to retain its full capacity! They also can only be recharged a limited amount of times (20-50) before losing most or all of their capacity.
All of these drawbacks make Ni-Cads less than practical in many applications. Enter the Ni-Mh battery! Not only can they have as much capacity as any alkaline primary cell, (upwards of 2700 mAh for a AA) but their internal resistance is even lower than an Alkaline , meaning they can deliver a higher current! They also have negligible memory effect and can be recharged 500 to 1000 times!
They are also not that expensive. If you shop around you can find top capacity Ni- Mh batteries for $2-3 each in quantity. If you figure in an inexpensive charger and you buy four Ni-Mh batteries, they will have paid for themselves before the time you would have gone through your second pack of twenty four Costco Duracell AAs.
The only minor drawback is for seldom used battery powered items. Ni-Mh batteries will self discharge even if not used, resulting in low batteries when not charged for a few weeks. Luckily with almost no memory effect you can charge them anytime without first first discharging them. For your less often used items such as a flashlight that sits in a glovebox or drawer, or for say a clock that usually runs over a year on one battery, there is a new technology called the Low Self Discharge Ni-Mh.
Ni-Mh batteries can take a few charge/discharge cycles to reach their peak capacity and it is recommended that you "refresh" your batteries periodically by putting them through a few full discharge/ recharge cycles to wipe the residual memory effect that will build up after many partial discharge recharge cycles. Every six months to a year should be fine.
Most of your batteries should easily last several years, although in practice there are always a few that konk out early. One of the major hazards is running the batteries so low that they cannot be recharged at all or even that their polarity is reversed! So when your device is displaying the symptoms of low batteries, pull them out and charge them, don't use them into the ground!