Haven't heard a single critical analysis of Thunderbolt. So here I go..
First things first, Thunderbolt is just a technology, not a protocol, which is good, but it also has some implications. For instance, it cannot really replace any existing protocols. So questions about the fate of USB 3.0 in the wake of Thunderbolt sound misguided to me.
It seems to me that Apple's intention behind this technology is to have a single type of port to connect everything. That would make it seem like you can get rid of your USB, firewire etc. ports. But since your device expects to be plugged into a certain port, you ultimately have to provide that port. Which means that you do need to buy a new dongle, unlike what people are claiming. For instance, if you have a USB flash disk, one end of the Thunderbolt cable can be plugged into your MacBook, but the other end has to expose a USB port (assuming Thunderbolt will carry USB packets, which it does not today). So either the cable has to be Thunderbolt on one end and USB on the other, or the device needs to have a Thunderbolt port. This is good for Apple because eventually they can have only Thunderbolt ports on their Macs, which saves money and makes them cheaper, but pushes the cost out to consumers or device manufacturers.
Also, currently supported protocols are PCI Express and Displayport. PCI-E is interesting because it makes it sound like you can in theory connect just about anything. But there are a couple of problems with this. If I want to make my USB device play nice with Thunderbolt today, I have to have something that understands the PCI protocol. That means, my USB device needs to carry an embedded USB controller. Similarly, if I have a SATA disk, it needs to carry a SATA controller. As one can imagine, this is expensive, not scalable, and just a waste of money. The other problem is that Operating Systems and BIOS's don't expect their PCI devices to get hot-plugged, which in theory they should, and Apple can certainly ensure that for the MACs. But most PCI devices themselves do not expect to be hot-plugged. Given these two problems, I don't see how an ecosystem can develop around Thunderbolt. But its Apple, so they might have a brilliant plan that I cannot imagine myself.
I do see this technology as ideal for docking solutions. Imagine your iPad or iPhone with a single Thunderbolt connector that can be docked onto something which can then connect to a mouse, keyboard, display and what not. Beyond that, I am running out of ideas.