I know there are folks who will swear by their rechargeables, and sweat at anyone who differs with their opinion, but opinions are simply that. I am not saying anyone who bought an XLT or vision dump their rechargeable batteries and charger. If you got it, use it, but be certain to keep the AA battery pack handy for times of need.
With all rechargeables there is an expense. If you pay full retail or get a great deal, it is still an expense so factor in the costs down the line. I am not simply referring to the cost to acquire them, either, but the convenience or inconvenience that might occur.
There are a few simple differences between using quality AA alkaline batteries and a quality set of rechargeables. One is the operating time. I have always been able to get longer operating time from a set of good Alkaline batteries. Matter of fact, several years ago when I was doing so serious battery comparisons I found a foreign brand of "heavy Duty" batteries that were nearly as long lasting as the Energizer alkaline batteries, for a much lower cost, and they also lasted longer than the various rechargeables.
There is also the convenience factor such as when you're off on a four-day ghost town trip. When you're trying to enjoy yourself and batteries die or get low, you can't plug them into a current bush. You need to have back-ups.
The main problem I have with rechargeable batteries, in addition to the shorter life/run-time, is how quickly they can die! Nothing worse than having batteries that check OK and then be off detecting and get a ways away from the vehicle and all of a sudden have them decide to take a nap ... and they do. They don't give you much warning when they are about to croak. Good alkaline will reflect their weakening condition better, and they don't have such an abrupt 'shut-down' as most rechargeables.
In the end it is one of the more personal things with regard to the metal detecting sport. Some like them, some don't. Some can justify the expense, and others (most, probably) won't use them enough to really make a difference is savings. You can probably get a better deal then those of us who shop at Costco, and in that case I'd just stick with alkaline batteries.
I buy the USA made Kirkland (Costco's brand) AA alkaline in a 48 pack for about $10.49 or $10.89, whatever it went up to. Divide that by 6 and you get the cost for a full set of 8 in the battery tray. With 40ish hours of run-time with the M6 and probably 50 or so with the IDX Pro, that makes detecting a very low-cost hobby.
Just my thoughts.
Monte
