I have no experience with the PMD661, but I do own the Tascam HD-P2. Leaving aside the mic pre-amps (which are OK but nothing special), the HD-P2 is a very good sounding machine. It does go through AA batteries at a good clip, but can also be used with external battery supplies, which can be rechargeable. I usually use the HD-P2 in the field with an external mixer of some sort, which allows for using a second recorder for redundancy.
The HD-P2 is not known for being reliable--I have sent mine back to the factory for repair one time, and I know of other people who have had problems with theirs. This is unfortunate when you consider that this a flash recorder with no moving transport parts. One suspects that the overall build quality is lacking. My solution is to always have a backup recorder of some sort along, and to use other recorders on jobs where I do not need the sound quality and features of the HD-P2.