Brothers, we have used the ID with great sucess during our training evolutions. You do have to be sure to run the rope backwards and if you run the slack end back over the ID - like when rappeling- it works very well and lots of control. I learned this technique at FDIC during the High Rise and sacffolding emergency HOT class, Mike beacon ex FDNY Rescue 3 was the lead instructor.
The ID is very versitile, just gotta train with it. I like the fact that I can go from a lower to a haul very fast. Don't have to chang my owering system much, just add another pulley and Gibbs to haul with a Z drag.

We have integrated the ID into every aspect of our rescue systems because of the versatility and safety of the device. I just tested an ascending system integrating the ID and it worked great. I am sure I am not the first to do this but I found that ascending with it is effortless. Even for a 220 lb out of shape boy such as myself. The advantage to using it for ascending is the fact that you don't have to do a changeover to come down. Simply remove your ascenders and pulley and go down. As far as control, when someone says stop, you let go and that is where it stops. If your having trouble stopping where you want, practice more, you will find that the possibilities are endless. You will learn to control your speed with the tail of the rope and not the lever.

Good Morning Brothers, I am part of a Special Operations Rescue Team here in WA. The Peztl ID was introduced to me about 2 years ago when my Capt. came back from a rope rescue class in Joshua Tree, CA put on by Peak Rescue Institue. I took the Technical Rescue course in October of '09 and plan on taking the Specialist Course this year. For those of you that are not familiar with this Organization here is a link to their website. http://www.peakrescue.org/whoweare.shtml. The reason I talk about this organization is that when they introduced the ID in their class they first went out and every member tried this out for themselves. This organization is made up of rescuers from all over the US from NY to Beverly Hills. They came back together and talked about the advantages and disadvantages of the ID. What I found particularly interesting is when they present any new tools for rescue, they talk about the products limitations, capabilities, history and how you can defeat it. Like Ryan said above, yes the ID can slip when it is side loaded. But this tool when used according to the instructions shouldn't be side loaded. One because the load to anchor should be along it's long axis and two the running end should always come back on the rolled lip of the tool. We use the ID (G rated) in a SRT for Rope Rescue and Tree Rescues. In a Rope Rescue situation we use it to 1) rappel single and rescue load 2) In a RAD system, 3) in a rappel to a pickoff system and 4) used as an adjustable pulley in the bottom end of a guiding line operation. We love it and are planning on introducing it into our fire department. I am actually working on that right now.
Just my 2 cents worth