I have had a quick look at this so I hope I havne't missed anything, was looking at buying OpManager so thought this would be natural to use as well, we currently use Cerberus,
http://www.cerberusweb.com and there are a few features of this that I would need to see in ServiceDesk.
1) Time tracking, already mentioned this one, but it is essential to able to enter time spent.
2) Multiple queues, as I see it at the moment to you have to run muliple copies to service multiple queues ? We currently run about 12 queues off one copy of Cerberus, each one with it's own Email address, teachs are in groups and can only see the queues that they have permissions to see, this works really well, most Helpdesk software support this, ie Perldesk etc.
Having said that I found lots of things I liked, if the above was incorporated I would seriously consider a move, ie the use of past soltuions to solve a current problem is way better in ServiceDesk. Still I am going to give it a good work out over the next few days.
One question for both ServiceDesk and OpManager, can they co-exist on the same server ? Will they set up 2 instances of MySQL ? Can they co-exist with an already existing MySQL server on the same box ?