It looks nowhere as good as MRI - but has the advantage of being very quick to do.
MRI will also image different things (depending on settings). US will show you interfaces between different kinds of tissue (that's where you reflect part of the US beam - echo!), but MRI has a million settings - and will fundamentally show you water at different densities. What that means in practice depends completely on the acquisition sequence.
The use case in my thesis went something like this
1. Do MRI - use this to plan surgery and to navigate while in surgery
2. Once the patient is on the table, register the MRI image to the physical location of the patient.
3. With various 3d tracked tools, you can then "point" at things in the real world, and have it show up on the MRI image.
4. As you are performing the surgical operation, you want to check "how does it look now?" - and that's where the tracked US probe comes in.
The use case outlined above isn't cost-sensitive, but time-sensitive. Getting an US image takes seconds and can be done mid-operation. Another point is that 3D probes are quite big and clunky - and for AVM-type operations where you crack the skull open, you want to keep that hole as small as possible. Phased US sensors + reconstruction then gives you 3D images with a very small sensor footprint. Another point is that 2D probes have higher resolution, so at least the XY plane looks much better this way.
