I could go on for hours about it, but the rex is the best canine toy I've seen in terms of proportions and general quality, including lifecasts (those have novelty factor but as far as toys go they're not always ideal)
The shaft itself has a bulging taper that can be fun to play with on its own, and if you're slick and relaxed enough will pull itself in after getting halfway up, which can feel great. The length of the pre-knot shaft is great for toying as well, so even if you don't feel like working on the knot during a play session it's still a great toy to pull off the shelf. The knot is probably one of BD's more challenging, but that makes for a /very/ satisfying tie when you get it, and it's really not that bad as long as you play over time and use lube along the way. It's a tight tie that won't just slip out, so it's wonderful to tug on and feel from inside.
I have a medium and a large I've knotted at times, and even though I've been out of practice lately, I was able to work for about a half hour and take the large's full shaft (no knot) and it was still a great ride.