DD is 20 months old. We have started about a week ago, it is all very relaxed, and just going through the motions of making her aware of when she wee's and poo's

Each morning when she awakes and in the evening before her shower put pop her on the potty, we have had great sucess in the morning, with some huge Pee's she would fight a bit to sit, but then as soon as she does a huge wee arrives, she also like toilet paper and "wipes" as well

We have had two poo's as well, but as she is mostly doing them in the middle of the day we aren't "catching them" at the moment

We aren't in a rush to get her TT, but seeing as she showed interest at a friends house and she wee'd on their potty, we thought we'd give it a go, the best thing was I could buy the cheapest potty @ Target for $7.99 because it had three ducks on the front and she LOVES ducks

So I would say if the interest is there give it a shot, if not then leave it for a while, I don't believe there is a postive effect if forcing a toddler to TT if they aren't ready.