I've had a great experience with signing as well (although using ASL, not modified infant signs). At age two and a half, my son has a VERY extensive vocabulary and is very expressive. For example, when I said it was sunny today, he said, "Actually, I think it's gloomy now." because a cloud had passed in front of the sun. One day he told me "Mom, I would appreciate it if you would pick that up." when I had dropped something. He speaks very clearly and his vocabulary (both the amount of words he uses and the descriptiveness he uses in his sentences) are much, much further along than any of the non-signing kids in his playgroup. He also knows all his ABC's (verbally and in ASL), how to spell his name, his birthday and the birthdays of myself, my husband, and my daughter, as well as the difference between first, middle, and last name (and knows them for all four of us), all the colors (along with all their signs), all his days of the week and months of the year, and he loves to show people how to form their hands into the correct handshapes for various signs while he tells them how to do it. Right now he is talking to my 8 month old in another room and pretend playing with her and here's the running stream of what he's saying right now: "Now it is time to sleep. Good night, good night. Oh, did you wake up, Jaina? Are you going to drink some coffee? Noooo, babies cannot have coffee, silly. Babies only get momma's milk. You cannot have any coffee yet, Jaina, until you are all grown up. Aunt Donna's baby is going to come out very soon. She's going to be a little, teeny, itsy, bitsy baby. Definitely. We went to her baby shower at Aunt Glo's house, remember?. Did you see her presents? She opened them up. One was really humongous from Grandma. Do you want to sit on my lap?" Oops, I had to go check on them when I heard that part, I didn't want him picking her up. Anyway, people, including his pediatrician, and the other moms in our group constantly remark on how advanced he is with his vocabulary. Signing has definitely enhanced his vocabulary, his love of books, his love of songs, his letter recognition, etc.

Emma
Signs of Early Life