I've used one, and liked it pretty well. Here are my thoughts:<br>
- convenient to have everything in the same place<br>
- design hooks onto ergo easily, but can also be put on a mei tai<br>
- backpack has its own straps, so you can grab it and go when you don't want to take the carrier along.<br>
- it's fairly roomy, without being overlarge<br>
- the biggest drawback is that when it full and heavy, it can make it hard to get the carrier on.<br><br>
Hope this helps!
I didn't particularly like it much. I found it really shifted the center of gravity of the Ergo, and with even a moderate amount of stuff put in it, made it very hard to get the whole thing up and over dd's rear end, because of the darts in the seat of the Ergo.<br><br>
I wish they would come up with a better design to keep it more firmly attached to the carrier, because it has a tendency just to hang down. I also didn't find it particularly comfy as a standalone backpack.<br><br>
I bought one a year ago and use it alone as a diaper/on-the-go bag. I have used it twice to backpack overnight with my 30+lb. kid on my back in a Kozy Carrier. The backpack hooks on to a Mei Tai too <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/wink1.gif" style="border:0px solid;" title="wink1"> We don't own an Ergo, and I have had better luck with the Kozy, personally.<br>
Overall, I really like the bag, and it was worth it.