Symbian is bleech. Don't mention it again.
Maemo could have been very competitive if pushed properly by Nokia. Nokia had a great device and great OS at their feet already last year itself, but they didn't push it. And actually this Android has picked up real momentum only in this period. When I had got my hands on N900, Android felt like really not powerful compared to Maemo. While using Maemo I felt like I am the Superman, no mobile OS gave me this feel. Maemo with the most potential of all if pushed correctly by n0kia. Maemo is the OS which really gives us the feel that we are using some kind of netbook.
Then comes Android which has really picked up this year. Surely more open than iOS and Windows Phone 7, isn't it? Open in the sense of "how" you can do things not in terms of "what" you can do. That "what" is nearly the same on most of the mobile OSs. Whatever one may say, Android is the current OS of the choice with a variety of fones from different manufacturers available. Many complain this is leading to fragmentation, but I really don't see any problem in that until the fone manufacturers support their fone properly (which they don't, eg. SE - too late in bringing up updates). Android is the OS of choice now. Much like Windows has been for the people in Desktops. [not comparing their respective market shares]
Initial releases aka iPhone OS didn't impress me. iPhone (ignore 4G) has always been a sub-par phone on specifications. Although iOS has been the OS to revolutionize the mobile market with it's "slickness", give something new to us else we would still have been stuck on Nokia 6600++++++versions still running Symbian. <rant>Now when Apple had a fone which was good on specs (iPhone 4G), you have to fu(king use a bumper</rant>
Windows Phone 7 is like the initial release of iPhone OS. It won't pick up straight away, will SURELY take atleast a year. The main difference is, we will see phones from different manufacturers. Manufacturers won't have anything to do with the working of the OS. Microsoft has made sure that Windows Phone 7 won't suffer any kind of fragmentation. Really depends on what kind of use you are. Some are/will be happy with it, some are/will be not.
Edit: Just for the note, Android has been out there since only two years.