I have had good sucess with NiMh AA batteries in both an older Etrex Legend and a 60CSX. These had a setting which had options for Alkaline or NiMh so the low battery alarm worked right. The ones I have used were made by PowerEx. There are two types:
2700 mAh and the newer PowerEx Imedion 2400mAh which come pre-charged. The newer batteries apparently will hold a charge for a year.
When buying these batteries look for their mAh rating and whether they are precharged. The "pre-charged" usually implies that the also have the ability to hold a charge for a long time. The normal NiMh batteries will lose their charge even if they are not being used.
Also have had some experience with other brand names with good results. In addition I have tested some cheap NiHh no-name Chinese batteries which I bought on eBay. The test results indicated that they did to have their claimed mAh capacity.
When it comes to NiMh batteries it is not worth it to buy cheap batteries.