Laurent:

The thing that is frustrating about fillets is trying to figure out work arounds, modeling steps leading up to the fillet may be causing the geometry to have issues, etc.

Different packages handle them differently too so try not to get too annoyed that you cannot use 123D on your machine. I can promise you that someone using 123D will come into a situation that 123D cannot handle but MoI will.

We would run into the stupid modeling issues at times in my previous career all using Solid Works Professional. Work arounds would include: fixing the model in Rhino then reimporting back into SW to finish it; or using the free version of ProE (this one goes back quite a ways) then reimporting the model to finish; or sending the model to a friend or professional contact and have them look at the model at try to add a needed feature or fillet with their package and then reimport into SW so I could finish the work.

Sometimes I would just have to "explode" (for lack of a better word) the model into surfaces, fix or add the feature that I need as a surface, then try to restitch the surfaces into a solid again which even that required me to use Rhino or some other tool or even exporting it out of SW as a non-native model then importing it again and letting the auto-healing option work its magic.

Just knowing what I know about MoI I would stick with it as overall it is pretty easy to use and allows one to model some pretty awesome geometry.

Pretty much any 3D package can get you painted into a corner but with the MoI UI being so clean and simple I sure wouldn't trade it away knowing that a much more complex package that is harder to use and more time consuming to arrive at the same place can also get you in a bind with something as simple as a fillet.