I love that Invision accounts for the various ways a user may interact with your prototype's various elements (i.e., hover, click) and the natural progression that would come after taking that interaction (i..e, after clicking stay on the same screen, stay on the target screen, close and return to the previous screen). I also like the ability to share projects with outside users in a controlled way (password protection) and that comments can be left in-prototype on the specific element requiring editing and checked off once complete.
This hurts to say, but the UX is a bit hard to use sometimes:
1. You can create sections for your various screens to live in, like categories, which is great. And, you can drag your screens or assign your to different sections, as needed. Both of these features are great. However, moving the sections up and down the page can only be accomplished via up and down arrows, as opposed to dragging. This is fine if your prototype only has a few sections, but when you have a lot of screens for a large project, moving sections becomes daunting and takes a long time if you need to move one section from the end closer to the top.
2. When you add new screens, they don't automatically appear in the section you were last in. They're added somewhat randomly, at least for me. I've had times where the new screens are added to the bottom of the page. Other times, they'll appear in random sections and I can't find them. It would be nice if I could select the section I would like the screens to be added to before they just populate the page.
3. It's difficult to see which screen to link a hotspot to in the dropdown list when in "Build" mode.
4. When in Preview and Build mode, you can hit the name of your project to see all of the screens in the project along the bottom of your screen horizontally. The previews are so small, that for large projects it can be difficult to tell which screen is which as you scroll through.
5. Sharing is a bit confusing. You can share directly from a screen you're on, and you can share from the main project page. You can also see all of the sharing links you've sent out in a list. The difficulty is that the list tells you the Invision-based URL and what screen # the share was made on. It doesn't show you the actual name of the screen (i.e., CTA Test Screen), the actual screen itself, who it was shared with and on what date it was shared. So, if you've shared various screens with different end users, there's no way for you to tell which share link applies to which recipients. Also, you can't edit the sharing functionality after sharing (i.e., update a share link to be password protected)
It's a fast, easy way to share prototypes with clients that allows them to interact with and test the mockups, as well as, provide feedback instantly.