I have similar experience to Deirdre. I started Shape Note singing about 5 years ago and I love it but still have a bit of difficulty with the shapes as I am more familiar with standard notation but agree that it has become easier to do the intervals.
Co-incidentally I've just spent the afternoon updating the mailing list for our Shape Note Singing which will be taking place in Holmfirth, West Yorkshire on March 27 - anyone interested in joining us let me know and I'll pass on more details.