I also have quite a few of these as well, and they really are an interesting training aid...a cylindrical golf ball? WHAT BLASPHEMY!
I used these up until I got the AlmostGolf balls that I currently use. I find the birdie ball is good to indicate whether or not you get a clean hit, but it isn't as good as the AlmostGolf balls at indicating the direction, fade, draw, slice, etc. that is the result of your swing. These don't fly as far as the AlmostGolf balls either, so if you are VERY limited on space you can pick these up. I hit my 3 Hybrid about 60 yards with these if I remember correctly.