I've had a little experience with Inventor. (We have Inventor Series 9.) I've used MDT for several years, and we bought it with its subscription. When the Inventor Series came out, our subscription was automatically changed to that - so I've been getting Inventor ever since. But I'd never done much with it until a few months ago when I decided that I was going to make myself use it. I took formal training on MDT a few years ago, but for Inventor I've just gone thru the tutorials, etc. (If I can talk my boss into it, I'd still like to take the formal training for Inventor. I feel that there are probably many things that I'm not yet familiar with that would be helpful to know about and use.)
My opinion of Inventor so far is I like it! It seems easier than MDT and more intuitive or user friendly. I'm currently converting MDT drawings to Inventor, and am anxious to use it more. When Inventor first came out, it seemed to be more suited for presentations rather than mechanical drawings. But now it seems to have similar capabilities as MDT. (In fact I do have a question about Inventor which I'm going to post. That's the reason I came here when I read your message.) My guess is that AutoDesk is gradually phasing out MDT and Inventor will eventually replace it.
I believe that if you already use MDT, I'd recommend the change, but if you don't, like t-bear said - there's a lot to learn. Hope this helps.