I agree on themessagecenter.com. They are great. I actually spoke to the owner when I first was looking into the issue.
You can see a bit about what I was comparing here:
ADVICE NEEDED: Who Offers The Best BES / Exchange Hosting? | BerryReview
I used to use exchangemymail.com but their 200mb storage limit does not hold a candle to the 1GB given for the same price at themessagecenter.
Also themessagecenter does not have a activation fee of $40 like exchangemymail. They also have no cancellation fee. They also seem to have a better version of Postini that will allow you to get daily reports with the headers of all your junk mail instead of just the alert that exchangemymail gives you
The upside of exchangemymail is that they are fervent BlackBerry supporters but I experienced quite a few planned outages compared to themessagecenter where I have not had even 1 planned outage in the few months.
Also Exchangemymail offers a much nicer control panel based on SWsoft compared to themessagecenters homegrown control panel. This is more of a tie since there are a couple key things you can do with each control panel that you cannot do with the other.
Personally I tried themessagecenter out just because they do not have any activation fee so there was no real big risk. The price is also almost exactly the same at 18.95$
PM me if you have any questions