The size of the iPhone will change by itself measured 11.5 x 6 x 1.1 to just 12.4 x 6,5 x 1,7 cm. The weight of 136gr to 198gr bearable.
Only negatives:
1. Something "fiddly" Dismantling the batteries (caution against breaking)
2. No mini-USB but special cables
3. Battery power is sufficient (in my usage and iPhone 3GS) about the iPhone 1x 20% vacancy fully recharge. (But this should be absolutely sufficient for the average user.
Big plus: The battery has its own on / off switch (So it empties the battery only if you also want that)
From me definitely a buy recommendation with those weaknesses