Rally good points from the PPs.
We are currently working through TT along with MM. Both of my kids were having mental breakdowns every.single.day when it came to math. At the time we were solely using MM. At that point I needed something other than myself teaching the kids new concepts. While TT fit that need, I wanted the kiddos to learn from two different angles, if you will, and since we already had MM, that is what we began concurrently using. It is so far working well. There are still days with gnashing of teeth, but the TT intro is taking the edge off the pen and paper work.
Also, regarding the levels, I agree that they are definitely not even with your typical grade levels. That is a simple fix in just testing out until you reach the point of instructional level with your kiddo. My oldest is doing TT 7 and following up with mastery work with MM5.
Hope this makes sense and helps all of us moms, no matter which side of the TT line we stand. ;-)