We go when we have time and I use some signs with her regularly. All the reading I've done suggests it actually aids communication because it's coupled
with spoken language, it's not instead of spoken language. So, for example, a baby that signs "milk" and says "muh" is more likely to be understood and have their attempt at the word milk reinforced than one who just says the sound "muh" if you know what I mean. There are kids of all ages at our group and none of them are late speakers, some are pretty early speakers in fact.
Becky can now sign milk, more and duck.
She's a chatterbox too so it certainly hasn't affected her babbles so far