Anal stimulation very rarely gets me rock hard. I'm fine normally from what i have read many other men are the same.
I have this particular toy and i found it kept slipping out. I have been meaning to send it back but have never gotten round to it. For me i think it is too long and because it's shape it naturally wants to push back out as it's sort of under tension when fully inserted.
I bought this one instead.
https://www.lovehoney.co.uk/product.cfm?p=35053
For me it's a much better product. It's still difficult to keep in place as the neck doesn't taper very much but I only use it for solo play anyway so it's not too much of a problem.
As already mentioned it does take time to get the full benefit from a new experience.