59s are shaping up really well, but the numbers aren't there right now. Which is to say, I'm not seeing games. WoW forum talk says check in after 4.0.3a drops -- several players plan to return after that.
As for grandfathering, losing Zul'Gurub does two things for several classes. First, it loses BiS bracers. Second, it loses shoulder enchants. For caster shamans, we're talking a loss of 3 stam and 4 int for the bracers, and 18 spell power for the shoulder enchant. Hardly gamebreaking. Losing the Arcanum head enchants means another 10 stam or 8 of any other stat. In total, playing against grandfathered 59s will be like playing without a neckpiece. The optimizer in you will dislike the absence, but you won't notice the 2% difference it makes to your game. Playing with or without BoA gear can make a greater impact, and several 59s forgo BoAs. So grandfathering is even less important.
DKs are a quick way to see what the bracket's like; there are very few actual 59 DK twinks around compared to other classes. Roll one up and see what you think!