I just recently escaped from outsourcing. My advice to any "good-sized" company who is considering outsourcing their IT department, be sure you know exactly what you want, what you are really getting, and what you will be losing.
I know you will be told that you will be getting access to a large pool of skills and knowledge, state-of-the-art technology, etc. Do you really think that is what you will be getting? Shame on you.
The outsourcing company I worked for wasn't top-of-the-line technology. The client I came from and went back to were farther ahead. It wasn't due to what the outsourcing company brought to the table, but the people that originally came from the client. They were the ones that stayed on top of what was happening and coming.
When you outsource, if your people go to the outsourcing company, then you will lose a good share of their time to other clients. They won't be able to give you the focus they did before. They will be at risk for being let go and your support will come from off-shore and near-shore. You will lose the knowledge about your company that goes into good support.
If your people don't go, but are let go, then you get the "pool" of support with most of it coming from off-shore and near-shore. You will be sharing them with many many clients. You become just a number. The central focus on you is gone. You are now part of the party.
We all hear about the "great skill and great knowledge" from off-shore and near-shore. Sorry, but I didn't see it. It is the same in other countries as it is the U.S. You have really good ones, you have mediocre, and you have just getting by. Well, the majority isn't in the really good ones category. Believe me, I know first hand. All of them are nice that I worked with, and good to work with, but I couldn't just hand any of them a task and get it done. It actually took more of my time to work with a lot of them to get it done then it would have taken me to do it by myself. Where is the gain? I worked with people from India, Ireland, Mexico and Malaysia.
Don't forget holidays. Now you have to contend with other countries' holidays with your support coverage. They have a lot more than the U.S. does, and I do mean a lot more.
Like I said at the beginning - think very hard about what you think you are getting. If you have an IT department, then you'd be better off keeping it and taking care of your own interest. If you are too small to really invest in IT, then you would be the one to actually benefit from outsourcing because you would be getting more then you could provide yourself.