I've been using MP for three years now.
I'm stuck with my current implementation of it because it's a live commercial site and I don't have the skills to run a test version of anything or sandbox paypal so I can learn, risk-free.
What I would like to do is move the whole thing over to Upfront and to this end I have been studying using the plugin and have three sites successfully using this so site design and layout I can handle. In the next few weeks I'm looking for developer support to:
1) Get more than two variations of a product working in dropdown- currently limited to 2. Each of my products has 3 variations and my workaround is ugly.
2) Have a better solution for postal options- again limitations of MP mean I have an another ugly solution i.e. separate "products" that are really only postal variations (like Tracked Shipping).
3) Housekeeping: Optimising tables; clearing out unnecessary files and anything else that's obvious to the tutored eye. I have looked at doing this myself but have been worried about breaking the live site and not having the skills to install a backup to server.
In general terms the wpmudev plugins work well enough out of the box but there is essential missing functionality, particularly in MP, for commercial users e.g. proper accounts for customers so that they can see their postal details, edit them and see their old orders.
I've been studying and educating myself as best I can about site development but I can see there is only so far I can go with a live site so I need developer support to get the above addressed.