First of all - almost all the negative reviews I see here are wildly misinformed! You should know what you're getting with this software (unlimited free character models with textures - and 2 free rigs a week) - and it is an ENORMOUS time and money saver for those who need custom character models on a budget.
Let me clear a couple things up:
1. You can ABSOLUTELY export your characters as OBJ files and load them in anywhere
2. If you want to rig/skin your characters for animation you upload them to Mixamo - and get 2 free per week.
3. Mixamo also has free animation packs that can be downloaded to get you character up and running in-engine
All claims to the contrary are just wrong. So where are there additional charges:
1. If you need MORE than 2 rigs a week
2. If you want to purchase their custom animations
Now here's one thing to bear in mind: if all you do is export one unrigged character mesh (with textures) as an OBJ file - you've paid for the cost of this software EASILY. Any decent character modeller will charge many times that for just the model. If you add in a rig/skin and free animations you could pay for this software 30 times with one character.
The Fuse models in general look very good - nice topology and high-res textures. Pretty nice editing tools as well. Easy to work in the app, and updates have been consistently high quality. Mixamo also listens very closely to their userbase and is extremely gracious about feedback.
Now is Fuse perfect - no... it's still in development. The major weaknesses are in variation and editing. Specifically, it still needs ton of additional content: clothing, hair, accessories, etc. In addition, there are some un-editable meshes (like hair) that would allow for a lot more diversity if they were editable. I also wish it was easier to get started with a basic variation of gender/age/build/ethnicity... like some macro controls before digging in and customizing. Or even better would be automatic randomization within those broader categories.
I still find it a bit difficult to get nicely-middle-aged characters (especially women). They either get too wrinkly or too pristine.
The randomization features are helpful as well, but I wish I could clamp them within a range of non-cartoonish values sometimes.
I'd also like to see native substance export - or at least more export options to match various pipelines (like various PBR).
Finally, I think Fuse is starting to suffer from prosumer-focus. Namely, you have a million nicely named sliders for adjusting parameters, but it is getting cumbersome to navigate the options. Some power-user tools (key commands and modifiers for the mouse) may need to come along soon.
Overall, this is extremely impressive software that can save you a small fortune. I can go from idea to a lip-synched and animated character in Unity within an hour.