Having used a (company supplied) Blackberry for many years, I needed to find something to use when I retired. I chose Win Phone 7 for many reasons (Samsung Omnia 7 on Orange UK). I am very pleased with it, and have now used it in Middle East, Asia and various European countries -- without a single problem. Links well into my e-mail account on Offcie365. Looking forward to Mango.
Only issues I have found so far are: lack of Skype (coming), the need to 'dock it' with Zune to download podcasts and its inability to 'download contacts' over Bluetooth to my car's built-in car kit, which was no problem with my Blackberry.
Biggest problem I have seen (in the U.K., at least) is its lack of availability / visibility. I went into my local Car Phone Warehouse (U.K.-wide mobile phone shop) and the guy was very positive about Win Phone 7, but said I had to go to bigger stores to get them. In Manchester, I wandered into various stores (Orange, Vodafone, etc.) and in each store there were umpteen Androids, iPhones and Blackberries, plus numerous cheapies, and then perhaps one Win Phone 7.
Not sure how Microsoft (or more likely, the phone dealers) expect to sell any phones if they do not actively sell them (and only someone who actively asks for one can buy it). Even on their mail-order Web sites, usually there is an extra menu for iPhones, and Blackberries, but for Windows Phone 7 you need to crawl through the umpteen entries under 'smartphones' to try and find Windows Phone 7 devices.
A number of the guys who used to work for me swapped their iPhones for Win Phone 7 and ,without exception, they are all pleased with their decision.
I was in IFA in Berlin two weeks ago, and thought I could find Microsoft to look at Mango in more detail. After asking around, finally found them in the 'Windows Phone truck' in the garden -- about as easy to find as the phones in the stores.
My girlfriend has an HTC with Win7 and I am impressed with the GUI -- easy to set up mail and the sounds, speed and general user experience were great. Microsoft needs to get this on more phones. Having to use Zune to update is a chore and it does take a while to update however it has yet to crash at all.
Not having to use iTunes for audio is more than enough reason to move from Apple to Microsoft.
The problem with a lot of watchers is they assume what happens in the first three months reflects personal opinion of everyone. I, for example, was waiting and waiting for Windows Phone 7, and when I couldn't wait any longer, I got an Android phone. It didn't really fit my need that well, so I switched to an iPhone. Better, but still not great. Then three months later, the Windows 7 Phone finally released, but I can't even consider it without a heavy penalty for more than a year yet.
So yes, I do think a large percentage will consider it, and 44 percent doesn't sound too out of line. Of that, expect about 20 percent conversion so yes, it should be viable if not 'likeable.' What really drives this, though? Business purchases. Otherwise RIM would have died a long time ago. If Mango really is good (especially for business), don't be surprised if it hits closer to 90 percent conversion as businesses drop RIM like a bad habit for the Microsoft platform. That is a BIG IF though.