We definitely use rechargable "AA" batteries for our cameras' flash units and keep several rechargers with us at all times. Because we go through a ton of batteries otherwise.
This way, when the flash starts getting a little sluggish, we swap the batteries out and away we go.
We also have a traditional charger for the batteries and a "quick charge" which will juice those puppies up in about 15 minutes.
Initial cost is not bad and you get that back almost immediately.
Check Sams or Costco as they usually have a pack of "AA" or "AAA" batteries with a charger included to get you started.
As far as my wireless mic goes, we are still using "traditional" 9 volt throw away alkalines, but plan on going the rechargable route very soon.