We just had a very successful handwriting day with my 5yo ds (with gross and fine motor issues) using HWOT. We love it here.
I use just about everything that they sell. My kids LOVE the CD. They beg to play it, and they want the song to repeat over and over (and over and over:001_huh:). My ds does sing little bits of the songs to himself as he writes.
If I had to limit myself to only a few items, I'd say that we use the slates and lined chalkboard a lot, the playdough not so much. The wooden pieces were a fun novelty for ds, but since the program only uses them for capital letters, they have a very limited shelf life. I find the teacher's guide to be helpful in pulling together different activities for each handwriting lesson. However, if your kid can pretty much figure out how to form letters and discern handwriting conventions easily, you probably won't need the tg, especially if you just intend to have him get more handwriting practice. The beauty of HWOT, though, is not the worksheets or the style of the letters. It's all about the songs, large motor movements, the teaching tips and multi-sensory activities.
As I said, my ds (who is not quite 5.5) has motor issues, and I have seen quite an improvement over the past four months. I really recommend the program.