Ds is 2.10 and we're about to try again.
Tried a few months ago when he was happily talking about the potty and play-sitting on it and chose some Thomas Tank Engine pants and would talk about weeing in the potty and pooing in the potty etc.
All very excited until we put the pants on on day 1. Five minutes later he wet himself. I have never seen such shock and horror on a child's face.
Cleaned him up, new pants and trousers, went to nursery.
After a week of no joy (except a couple poos, because he tends to be slightly constipated so you get a good 10 minutes warning!), we gave up.
For the last few weeks he's been in pull-ups and been made to practice pulling them and trousers up and down, because if he couldn't get that it wasn't going to work. He will sit on the potty pre every nappy change, but I think is actively clenching and refusing to wee despite chanting 'if I do wee in potty, I get chocolate button'...
He's currently the only one in his class at nursery not using the potty or toilet. But the staff who had been convinced he was ready were then saying "I've been doing this for years and never met a child like ds before" - whereas I've at least been an intensely logical stubborn git like he is! 
I tried introducing him to the little toilet at John Lewis. He inspected carefully and said 'it's scary'. So I got a toilet seat insert and stuck it on his head to prove he couldn't fall through it, which he agrees. But doesn't want to use it.
Hopefully the weather will be OK this weekend for playing in the garden and peeing everywhere and cluing him in, as the plan is to try again on Monday. Thing is, he is the opposite of the 'I do it!' child - would love to be carried everywhere, fed, stay in nappies forever, etc, so I think we have to do what we've done with other things and just say no to nappies...
Sorry, that turned into an essay but I try so hard not to constantly talk about ds and poo etc in real life!