Awesome pack, love it love it love it.

The Deuter Kidcomfort III is my favorite piece of baby equipment, no contest. It is so comfortable and easy to use, I actually look forward to using it everyday. When my baby was smaller (we bought it when he was 8 mo. and just barely big enough to fit it comfortably) I would look forward to the part of the day where I'd strap him in and go for a long walk with the dog - it was my "myself" time - even though he was awake and totally entertained to be looking around! My son loves it - so much that sometimes when he's antsy to go out he'll go and sit himself in the seat (yep - the stand is stable enough to withstand a toddler crawling in) and whine to get us going. I was really worried, given how much I love the pack, that he would get bored with it once he became a walker, but he still loves it. I think that's just a testament to how comfortable it is for the child and what a great view it is.

One quick note: I note below that we rarely use a stroller because we use the DCIII instead, but I don't use the pack for long walks in paved areas - shopping excluded - because my knees protest from the impact.
If you live in a highly developed area and don't walk off pavement much, some of what I have to say may not apply to you.

Adjustment/size:
This pack is very adjustable so I would imagine it fits a wide range of people. I am 5'8" and my husband is 5'10" and we use the same strap adjustment in the back, which is somewhere toward the top of the middle, so I'd guess it fits people quite a bit smaller and larger than we are. With the Vari-quick system, strap adjustments are absurdly simple to make, so it wouldn't even be a problem doing it on a quick changeover mid-hike. However, a very small person might find the frame comes up above their neck, limiting head movement? You'd want to check before buying if you are concerned about this.

Padding for adult:
The straps are well-padded and a nice wicking mesh on the inside so I don't even mind using this pack during the summer. The frame and hip belt are very secure and ensure that almost all the weight is distributed over your hips and legs. I have never had back pain using this carrier, even after hours of use. On the other hand, if your legs get tired, there are 3 sets of adjustments you can make while walking to change the load distribution a bit and give your muscles a break. In fact, this pack is so comfortable that I volunteered to train with a friend to do a fitness test requiring you to carry 45 lbs for 3 miles in 45 minutes (a good clip). I just tossed two gallons of water in the bottom cargo compartment, and off we went! So I'm sure the pack will continue to be comfortable even as we reach the maximum weight limit (which is a few years off).

For the child:
The cockpit is padded in all the right areas (including the crotch) with a soft fabric and cleans up easily. The removable drool pad is a must - easy to remove and wash and also nice padding for when the child falls asleep (which happens pretty often). The padded 5-point harness makes him so secure that I can do almost anything with him in the pack. The side-entry makes this so easy and nice to use, and simple to close up and get going.

The part that makes this pack a little bulkier than the rest is the integrated weather shield - it means that the back rises up higher than other carriers. I love this part though. It means better head support for the baby all the time, and I like always knowing that the weather shield is there in case it gets really hot or starts to rain. It opens up simply and covers well enough for anything but face-on sun or a driving rainstorm. The greatest thing about this is it's always there, and doesn't get in the way or take up cargo space. Most of the time, you can't even tell it's there, and it folds away easily. This is a great design.

Other features:
As I mentioned before, the stand is totally stable, and so painless to deploy with the pack on or off, I can't imagine why an auto-deploy (like in the Kelty) would be necessary.

Lots of cargo room - enough to pack for a weekend trip! I use this as my "stroller" for gate check when we fly - and it's like getting a free piece of luggage in the deal.

The little pocket on the hip strap is perfect size for a cell phone or wallet, which is really handy for daily use.

Things I would change/add:
Deuter sells a baby neck pillow which I would consider buying since our baby sleeps in this pack quite a bit. It always looks kind of awkward, but it's been a year and he doesn't seem to have any problems, so maybe this is just my issue.

We keep a compact mirror in the side pocket because it is so nice to be able to check on the baby (or just play with him). Some carrier brands sell a mirror that hooks on for this purpose which might be worthwhile.

Summary:
We've owned this pack for over a year and continue to love it. We use it daily - for walking, hiking, shopping, outdoor activities, and my husband has even done some landscaping and construction work with it on. We rarely use a stroller because this is just so much more convenient all the time (and a better workout for busy parents?) Sometimes we even battle over who gets to carry it.

If you plan on using a pack a lot, I'd recommend this one hands down. We don't use it for serious hiking all that often, so you don't have to be a hiker to appreciate it. I've even started taking it shopping despite its size. If you are only going to use a pack for specific occasions, a less feature-laden one would probably suffice.