So in the end, the system is pretty solid, but the biggest advantage is reach. They reach players we wouldn't normally reach.
Yes. For instance with Tabletopia you will reach all the people using Mac OS X and Linux.
There were some statements form the OCTGN developers that they plan to create a Mac version but theses are more than a year old. So as a Mac user the only option is to use Wine here. For my system I finally managed to make it run with Wine but although I can use the spellbookbuilder and start or join games it keeps on crashing in the middle of my games, which takes all the fun out of it and prohibits to take part in games with people I do not know personally
For this reason (and also the overall more polished UI) I'm very excited about Tabletopia because I think this will enable me to play mage wars online at last in the very near future.
Please note that I really appreciate the work of the OCTGN team for making and keeping the work up on this great non commercial platform and also of sIKE, creating the awesome mage wars module but since it doesn't run on my system (yet) I'm very happy that there is an alternative which looks really promising.