HEAT is highly customizable. There are many options in regard to improving efficiency within the product itself, as well as an ability to call other products from within HEAT to increase funcitonality.
HEAT is also fairly cheap for an incident management system.
There is a pretty good user community currently - users helping other users.
In order to get more ITIL functionality out of the product, you either need to purchase other modules or a different product. I had to do some creative customization to shoe-horn Change Management and Proejct Management into our installation. Right now we feel like we just can't do anymore with the product as is.
Making changes to the system is an interesting process, unless you have experience with the system. If you don't do updates in a certain way, you could potentially cause 8 hrs of downtime instead of 1 hr - for the same change. Also, the ability to make some aesthetic changes "on the fly" is more likely to cause problems, so it's better to avoid it altogether.
The Self Service module was difficult to install. Once it's up and running, it's not terribly easy to make any changes to it. We eneded up building our own custom website instead.
Support is not great. Often when calling support, I felt like I was dealing with someone who had never seen my issue before. It had reached the point where calling the actual support line was my LAST resort. Also, it seems like the same product bugs keep showing up over and over as new releases came out. It was frustratingto deal with as an administrator.
If you don't have a lot of users (100 or less), and are just looking for a system to track incidents only, I think HEAT is a good choice.
If you want a product that better will all you to follow ITIL best practices, and potentially be used enterprise-wide, you should look elsewhere (and expect to pay more).