Eneloops are low self-discharge, meaning you can let them sit around without using them, and they stay charged. If you metal detect least a few times a month, you will probably charge your batteries more often, than the low self-discharge will make much difference. You can get higher capacity, and a lower price on other brands.
Fast chargers, and timer based charger tend to cook your batteries over time, reducing their useful life. Battery companies like this, so you continue buying batteries. A smart charger will get you years of use out of rechargeable batteries, well worth the investment, especially if you have other electronic devices you want to go rechargeable. Overcharging is one thing that kills rechargeable batteries, smart chargers eliminate it. The other thing that kills these batteries, is over-discharge. With disposables, we naturally want to get full value out of them, before the trashcan. We run them down, until they no longer power our devices. Rechargeables like to be charged, better to keep them topped off, than to let them go completely dead.
Rechargeables will probably never compare to alkaline, for runtime comparisons, or price. They will same you a lot of money over time though. I don't hunt more than 2-3 hours in a day, mostly weekends, and get more than a month between charges. Have never had to swap out batteries during a hunt. Usually charge them when the meter gets about half way. Should easily last several full day hunts.