A user of Motionbuilder since its inception under the Kaydara banner, I've always loved Motionbuilder for its laser focus and incredible real-time engine (That could run highly dense datasets in close to real-time long before GPU or Multi Cores became the order of the day.)
If you are doing high end or serious game Performance Capture, what's not to like about Motionbuilder? The first true non-linear cg animation solution, Motionbuilder has really added any new or significant improvements in 2016, more importantly, it has shared even more of its features with Maya and Max. Enhanced HumanIK tools in both suites have become easy and more robust with new solvers, all built on MB's HumanIK standard.
Today, several good tools are available on the Indie market for Motion Capture. Using a Kinect, software such as IPISoft and IClone can do awesome motion capture and some cases match or exceed what Motionbuilder can do. What's best about Motionbuilder is it's Performance Capture not just motion capture. It allows the artist to combine not only capture facial, body and re-target stock motion clips, it allows them to combine, trim, edits, adjust curves and graphs thus fine-tuning the animation to match the individual characters perceived personality. From Facial capture to tracking of high end Actor capture like Avatar (In fact Motionbuilder was the major pipeline solution for capturing the actors performances and transferring them to the 3D characters).
The Story Editor functions as a Full Pre-Vis/Storyboarding environment inside of the Animation software. If you are a Live Action Narrative Storyteller, this tool allows you to build virtual cameras in the sequence you want, place virtual actors and drop motion clips into the scene. This creates an easy Storyboarding tool for getting approval and sign-off on your scenes.
To that end, Motionbuilder is infinitely customizable via the powerful scripting language Python. This and a very accessible SDK, gives the user a lot of power in creating plugins and ports to it's open environment. Motionbuilder can support control from Industry cameras such as Vicon or OpiTrack or low cost options such as the Kinect. Motionbuilder can even be driven in realtime by MIDI, LTC Timecode or GPI Triggers (This allows real-time performance and driving of CG elements. Think a CG character interacting with a live person in a presentation (via triggers) or performing certain task via a MIDI input.
The killer option that puts Motionbuilder in a class by itself is it's support for Virtual Cameras. This allows the user (Director, Cameraman) to virtually control the camera in 3D space. Using tools such as Optirack's Insight VCS Pro with it's Motionbuilder plugin, an filmmaker can organically operate a real camera rig, while watching the 3D scene in real time. This bring unprecedented realism and creativity to any project.
Motionbuilder has seen a substantial upgrade in years. In fact, most of it's core functionality has been duplicated in Maya, so while it is still king for working with FBX and assembling and editing layered animation, task such as re-targeting, quick rigging via HumanIK have long sense been in it's bigger, more famous brethren.
The workflow for dealing with BVH motion capture needs to change. Back when FBX was a fledgling file format that Autodesk needed to promote over the industry standard, making an artist import a skeleton, rig it then re-target to your asset. FBX is king now, BVH motion files should be drag and drop and work in the Story Editor.
In the Story editor, Autodesk should match the Story environment in Maya thus making direct connection of Storyboarded Story content immediately available in Maya.
Lastly, the UI needs an overhaul as it hasn't changed much since it was in the hands of Kaydara. (This won't happen, but I'd like to see it).