Windows Phone 7 Mango is moments of being released with the RTM version of it already given out to manufacturers. Only time will tell of when users will be able to upgrade their devices to the latest and better version of WP7.
However, some people may still find Mango incomplete in some ways. For example, the calendar does support multiple sub-calendars for certain providers but not for Google accounts. I for example, have 3 to 4 sub-calendars on my Google account but only my personal calendar is shared but not the others. This can be quite troublesome for many as well.
In addition to that, the lack of free and good apps is pretty disheartening. Being an ex-Android user, it is very noticeable as it is hard to find a good app for the Windows Phone 7. Maybe time will tell. Hopefully, games such as Angry Birds and such will be made free like they are with the Android platform albeit with ads.
Multitasking support is also subpar as compared with platforms such as Android and iOS. This is because not all apps have multitasking turned on by default. The developer needs to utilize the feature and update the app before it can be used on a phone running on Mango.