I'll preface that I have the iUp as a backup, not my main battery pack. I only got it for a trip I was taking to the MotoGP in Indy last year. This info is also a year old because that's when I was buying these things.
I prefer battery sleds to external packs as they also act as a hard case. I found it far more convenient not to have to carry a separate case with me. The extra weight of the battery guarantees that in a drop it will most likely land with the screen-side up. With a sled I don't have anything extra hanging off the phone and can use them while making calls, walking, etc.
I tried the Incase battery pack first and didn't like it because it was significantly thicker (it was a higher capacity battery, but not enough to justify the significant increase in size), had a matte surface with made it hard to pull out of pockets, and it didn't have an on/off switch. A year ago there weren't other battery sleds that were a comparable size to the Mophie. The only things I would change on the mophie would be greater capacity (duh), more LEDs in the battery level indicator (I wish it had eight or ten like my MBP instead of just the four it has), a raised lip like the Incase so setting it screen side down wouldn't have it resting on the glass (a screen protector solves this), and something to protect the back of the iPhone from scuffing when you remove the case.
The Mophie uses Micro USB rather than dock connector. This never caused any issues and allowed syncing and charging, but it just feels inherently weak and I always felt that over time the jack would deform a little and get jiggly and loose. Obviously docks, accessories, and car kits are out with any sled.
Sleds also have the advantage of keeping your battery in mint condition as you are hardly ever hitting the internal one. I just left the Mophie on and let it drain before moving over to the internal. Its excellent piece of mind, especially if you jailbreak as you can use all your phone's features all day long and never have to worry about running low. You never have to leave the house and worry that you forgot your external pack.
