The majority of OU may be immune to Toxic Spikes, but like many others have said, it can be very helpful against certain walls, Bulky Waters, etc. The important thing to consider is whether it's worth dealing with the immunities to poison those one or two Pokemon per team that will really be crippled by the effects. Sure, maybe their Scizor and Metagross will be immune to them, but that might be worth it if you can poison that Hippowdon they've been hiding backstage, or set up an easy kill on their Tyranitar.
Also, like Spikes, I think it's significant to notice that a large number of the Pokemon that are immune to TSpikes will take significant damage from Stealth Rocks. I think the key to using T-spikes well is to use them in cooperation with not only walls, but Stealth Rocks and/or Spikes, as well as a Ghost-type (spinblocking and Rotom/Gengar check) and viable counters to the various Immunities and Steel-types that will resist both. (I'll take this oppurtunity to nod in Infernape, Heatran, and Lucario's direction, since they can handle a lot of the typical Steel-types, as well as Immune and Natural Cure Pokemon like Snorlax and Blissey.)
It's not as much of a universal hazard as SR or spikes, as you actually have to build a good portion of your team around them. But that doesn't make them useless by any means.