I have Mandrake 9.1 and last night I managed for the first time to get my WInmodem to work now I wonder should I upgrade to Mandrake 10?

Yes or NO

Your reasons why

why not? go for it dude. mandrake 10 has much better support for hardware since it runs on the new 2.6 kernel

Mandrake 9.1 is very stable. Mandrake 10.0 isn't as much.
But I recommend mandrake 10.0 because it has the latest software. For example 9.1 has openoffice 1.0 and mozilla 1.3, while mandrake 10.0 has oppenoffice 1.1 and mozilla 1.6.
Mandrake 10.0 also has an updated KDE desktop and the new linux kernel.
Personally, I would recommend 10.0. But if you think winmodem won't work with 10.0, stick with 9.1 until 10.0 is updated. But, if you have the drivers for winmodem, go ahead and upgrade to 10.0
I came upon this site. I donno if it helps http://www.linmodems.org./ What winmodem do you have?
One more thing about 10.0, boot with CD2 and insert CD1 when it asks for install CD

what version you're talking dude, the community version or the offical version? mandrake 10 offical is very stable and you should not have and problem booting from cd1. Remeber the community version has bugs and is not the offical version.

If you want mandrake ten get the offical version.

I'm using the community version. I didn't know the offcial was out.