We bought one when Ellie was 18 months. We showed it to her and left it in the bathroom and I sat her on it before her bath each night. I didn't get a single wee in it til she was 2 and 4 months.
. I don't think it does any harm having a potty around the house but I don't think it speeds the process up in anyway.
Ellie was really difficult to train compared to me other two but in most ways she far more advanced so I was surprised. I'm now convinced that it's nothing to do with how bright they are etc it's to do with when their body and brain come together and they're ready.
Im also convinced that Ellie would like to wear nappies forever as shes such a lazy moo :-) xxx