It's 10 years since I did baby signing with my DD, but back then all the research indicated that signing helped children with their language development and did not hold them back in any significant way.
I signed with my DD. We didn't attend any classes etc, but I just incorporated signing into every day life. DD was a prolific signer and then speaker. At the time, she was also being observed by the babylab for her language, and so I had to document for the research all the words she used. At the age of two, she had over 500 spoken words with the vast majority of them being words that she had initially signed. The average for 2 is 50 words.
I think the thing we got out of it was the fact that she was able to convey complex ideas. at 12 - 15 months, she could ask for me to read a book to her, and then tell me which book she wanted. (she'd sign book followed by bird, for the bird book etc). We also discovered the reason why she was waking at night when she signed light in the dark. This was at 11 months. We worked out that she was scared of the dark, turned the light on and weeks of night waking vanished overnight! It also helped to overcome toddler tantrums because she was able to communicate exactly what she wanted, and from 12 months onwards was able to combine signs to form more complex sentences. She was able to ask for things that was beyond her verbal capacity - for example, she could ask me to put the washing machine on her (she used to like watching it!) etc.
For us, I would say that there were no negative consequences but you have got to make signing a natural part of your language for a few years. You have to get used to just signing every time you speak. Do that, and they pick it up quite easily ime.