The P990i is still a little buggy but stable. Though there are less 3rd party applications on it, but all that you need for a business phone is there... schedule, phone functions, opening Word, Excel, pdf documents, camera, etc.
If you are looking for a serious business phone, then the P990i is a contender. If you are looking for a PDA phone that you would not mind little problems here and there, but want to download a load of 3rd party apps, then go for the Dopod... good luck with the Dopod.
If you are looking for a business phone that is stable, has reasonable amount of 3rd party apps, sounds great as a phone, does email with Blackberry support, opens Word, Excel, PDF files, keeps your schedule, sync with both PC and Mac, then you might want to look at the Treo 650 or the newer 750. So far this is the most solid business phone I have ever used.
Did I mention FAST as well? The P990i is slow. Maybe SE will fix that with firmware updates. So its up to you really... go test the P990i and see if you can stand the speed of the phone. The same problem is with the M600 as well... Symbian 9 is slow... painfully slow...
I will never part with my money on the Dopod. Hell, I will not use it even if I get it for free... I know many others will... but I really cannot trust the M$hit operating system no matter what they say.
The above are my personal opinion and preference. In no way this is meant to offend any Dopod fans or Windows Mobile user. After going through all the PDA phones in the market without a budget in mind, I still settled for the Treo. After sales service is still one of the best in the market.