USB battery packs

Since I can’t find decent mp3 players with an easily swappable battery, I have been focusing on USB battery packs. I have the Tekkeon mp1550 which uses 4 AA nimh batteries, and the Duracell Instant Power Charger

and Lenmar Powerport Mini which have a built in Lion battery.

I am very happy with the mp1550, which works great with 4 nimh AA batteries. While it can be used with alkalines, the charger can become very hot with alkalines, and wastes much of the power in the alkalines. Using nimh batteries the charger doesn’t get hotl. It will also charge nimh batteries in it via USB9however it takes around 6 hours). The mp1550 has a built in voltage regulator(which many AA USB chargers lack), and a power switch(to avoid power loss while betteries are stored in the charger). 

I recently bought the Duracell Instant Power and have not tested it yet. It received  good reviews.

I would appreciate comments about other USB chargers. I would love to find a USB charger similar to the Tekkeon mp1550 except that it uses(and charges) an easily swappable 18650 Lion battery(or perhaps two of them at a time?) rather than AA batteries.

The Tekkeon mp1800 with its 4,000 mah built in lion battery looks interesting, as does the Sanyo KBC-L2S.