I have a Tekkeon TekCharge. Its advantage is that it uses four standard AA cells, so you can "refill" it anywhere you can buy batteries. In effect, it makes your phone an AA battery device. I always use rechargeable AA's in it, so I'm not using disposable batteries.
Its drawback is that it just "balances" the charge between the phone battery and the AA's. If, say, the phone is at 50% charge, and the AA's in the Tekkeon are below 50%, the phone won't be charged. In fact, it seems that if the Tekkeon's AA'sa are at a charge level below the phone, the power will flow the wrong way. You can keep the phone charged at whatever level the Tekkeon's AA's are at, but the Tekkeon can't act like a "pump" to squeeze power out of its AA's and into the phone. The Tekkeon with rechargeable NiMH AA's can charge the phone the equivalent of 2-3 times.
I find a much better solution is a spare or spare batteries for the phone itself. They are compact and light. Probably only 2-3 of them store as much power as the Tekkeon with one set of rechargable AA's, and several phone batteries are far smaller and lighter than the Tekkeon. But - you do have to recharge the phone batteries eventually, while you can just buy more disposable AA's for the Tekkeon.
As WestCoastPaddler said, another option is the battery packs. Which you choose will depend on your circumstances. You might also look into solar chargers.