moedertje,
In the younger years, it's not a problem to use one math program as a supplement to another. As kiddos get older, however, it can get to be pretty time-consuming to do that. So, you'll want to stick with one main program at that point.
As far as Teaching Textbooks goes, we haven't made any firm decisions yet. We have heard good things about the Teaching Textbooks Geometry, so it is for the Algebra I and II that we will most likely choose a different math path to follow.
We have looked into Math-U-See for the upper years, and feel that there are better options for the high school years that we would recommend first. Math-U-See appears to rely heavily on the blocks used in their program and doesn't seem to make the shift as easily to higher level problem-solving.
I did want to update this thread to say that we have since ordered and taken a much closer look at Kinetic Books Algebra I. We found that while it is very thorough in its explanations, it is a bit overwhelming in content and also in its liberal use of math vocabulary (to the point of making a simple equation seem difficult to solve). The printed copy of the text is actually as thick as a telephone book with a very small font. While the print copy is not actually necessary to make the program function, we did find that the number of problems expected to be completed is very excessive. It would require a discerning parent to determine how many problems should be assigned out of the very large problem set each day.
We have also ordered the VideoText sampler and printed off the corresponding lessons that go along with that program. At this point, we find VideoText to be easier to follow than Kinetic Books, with a better system of checks each day to make sure the child is truly understanding the concepts. Video Text certainly isn't flashy, and I know my son will miss the tongue-in-cheek humor of Teaching Textbooks. However, the math presentation in VideoText is very solid, and students take a short quiz each day to make sure they've retained the material BEFORE moving on to the lesson.
The explanations on the DVD's are very thorough and make a lot of sense (even for the non-mathy parent or for a parent who hasn't had Algebra in 20 years like me!) Students work 10-11 problems each day, and if they don't pass their quiz, they work the other half of the problems the next day (after watching the DVD lesson again). There is a help-line for kiddos who don't pass a quiz after 2 tries, and additional problems available on the website for each lesson if needed. Last, there is a line-by-line solutions manual for ALL problems, meaning kiddos are responsible for error analysis of each problem to see where they went wrong.
Much of this program can be done quite independently, although I think it will be a good idea to watch the DVD Lessons with my son (as they are each only 5-10 min. long). At this point, we are leaning in the VideoText direction for next year. I'll link you to the professional reviews for this product from people I definitely respect as to their professional opinion:
http://www.videotext.com/testimonials.htm#prof
We are continuing to order and research other math options in order to make a choice for next school year for our own son.
Blessings,
Carrie