We have an ex-racer TB who for the past 8 years has been used as a brood mare and not ridden. She's just foaled (4 days ago - so proud of her - hehe) but hasn't been ridden since last summer. I plan to wait and apart from lunging (to which she's an absolute dream) am going to get her re-backed next year. The only reason I'm getting her re-backed is that she doen't like you using your legs whilst riding her - a TB rescue centre said that jockeys don't use leg commands (due to the way they sit) so she's never been trained to ride that way. I know i'm a decent rider, but don't have the experience that she needs/deserves, so i'll leave the training to the professionals. She's a sweetie (most of the time) but we have to be firm with her when the hormones kick in!
Her weight was a problem - we just couldn't keep it on her - despite her being on the best available over here, but then I started to add a little corn oil to her feed and added sugar beet - that's been the magic formula for us!
She's 14 now, and being an ex-racer has seen it all - she's flown, been prodded and poked and doesn't mind most of what we do and doesn't spook at anything (except umbrellas, but we're working on that haha).
So for an instant riding horse for a family, an ex racer TB isn't probably the best choice, but for us who have the time and patience (and hopefully another 15+ years with her), and overnight result doesn't matter - we're happy to take it slow........