i bought my dd a potty(and an activity table so that she would stay seated) when she turned one cuz i could tell almost all of the time when she needed to poop. whenever she needed to poop i would put her on her potty. my sole purpose was to save her(and me, lol) from poopy diapers and it worked. she is now used to sitting on her potty. in the process(months later), she learnt that she could also pee in it, so now i put her on it every morning when she wakes and we save another diaper! at 17 months, she is not toilet trained and i don't expect her to be but we're half way there

. the benefit for us is that she knows what the potty is for and will try to go when i put her on it. the bonus is i've saved some diapers and no longer need wipes. i'm also glad that she no longer has to have poop squished against her precious skin.
i don't think it would hurt to start training now at 15 mos., especially if she is interested. i just wouldn't expect her to be 'fully' trained at such a young age.