Prior to the iPhone hit last week I set out to switch to IMAP mail. Before I worked in ITS I was over in the College of IST ... for better of for worse (mostly worse) we were all on an Exchange server. There were problems, but I have to say I had missed the mail functionality since coming over a year and a half ago. I missed having an identical view of my mail no matter where I looked at it. I am not a fan of webmail -- of any sort, not just PSU's -- I am mail client kind of person, so that's never been an option for me.
I had settled on dealing with the situation by only ever checking my mail from one machine -- not something that is all that feasible given the fact I have a few different machines. When the iPhone pilot became a reality I knew I needed to get off POP and onto IMAP. Hence the switch. I have to say, the PSU IMAP service is rocking! My mail is always in sync, I never get the annoying error messages I was getting before, and I can feel confident that if I am away from my MacBook I can grab email. IMAP at PSU carries a small fee, but it isn't all that expensive.
IMAP on the iPhone is the only way to go (IMHO) ... I like to get mail on the go and have its "state" be identical when I get to the desktop. I only wish I had my MailTags plug-in on the iPhone, but you can't have everything. At any rate, email on the iPhone is a pleasure and having my mail synced up with my desktop is wonderful. I am checking and sending more and more mail from my iPhone, so having it all work so nicely is very cool. If you are going to go down the iPhone path, I highly recommend switching to IMAP.