At least for me. Had it at my last job.
It was a great gadget, but the core phone performance was iffy. Sound yucky, poor microphone, etc. Bluetooth headphone was a must. The email and instant messaging was fine.
This job I've currently got a Samsung Blackjack (oooh, pretty), but never enabled the email since our corp internet plan is very expensive. Having just determined I need email, I'm waiting for my Blackberry.
I think the Treo is painfully bulky compared to the other choices, but the user interface was very good.