Dual-layer cases typically include two separate pieces that are used together to create the extra levels of protection. The Speck CandyShell for the HTC One is different in that it's made from two layers molded together, something which is commonly referred to as a hybrid case. The CandyShell features two distinct fused layers. Hybrid cases are an interesting beast for the very reason that they typically fall somewhere between a single layer case and a true dual-layer offering.
The exterior is your average hard shell with a glossy finish. The back will pick up fingerprints and dust, so if that's something you're averse to now is the time to back out. On each of the four corners you can see a small strip of the TPU material that covers the interior. What you basically have is a thin layer of TPU built on-top of a hard shell. The CandyShell is a hard case but it's not dual-layer hard. You'll find that it performs well in everyday environments and construction wise is all most people will actually need.
TPU covers the volume rocker while the rest is hard shell. The major downside to the CandyShell, and most hybrid cases in general, is that it's difficult to get the One in and out. The CandyShell is the type of case that works well as long as you don't need to frequently remove your One, as getting it out is not a pretty picture. The TPU and hard shell form a tight grip on the One which some may see as a plus. It certainly won't pop out easily in the event of a drop.
The Speck CandyShell for the HTC One is a solid hybrid case that simply has all the faults that hybrid cases tend to have. Average users will get their money's worth when it comes to protection, though the glossy finish will attract fingerprints. Getting the One out of it is a chore, but you'll be good to go as long as you plan on keeping it in there.
You can pick up the CandyShell by Speck for the HTC One at the HTCpedia Shop. You can also pick up other HTC accessories right here.