This is an issue that quickly gets amusingly complex. The short answer is that there are a myriad of ways to fit 3.5" HDDs into 5.25" bays. However...
As someone else already mentioned, the 5.25" bays in most cases are dead zones for air circulation. Optical drives generally don't need it, so there aren't any fans in front or behind the bays to move air. For a one-off HDD this is generally fine (especially if it's a 5400 RPM drive), but if you're going to install multiple drives then you will want something providing airflow.
Which brings up larger whole bay adapters like the 5-in-3. These come in many designs, everything from simple metal racks to complex tray loaders. The important part here is finding something with a fan, and then figuring out how much you're willing to spend. The tray loaders are nice, but they are relatively expensive. Either way, you'll want to go the route of a whole bay adapter with a fan if you're looking at installing multiple HDDs into your 5.25" bays.
The Icydock technically doesn't fit. Its design doesn't account for the small rails along the bottom edge of each bay position used to hold up a 5.25" drive. Which is why there are so many comments about bending the rails.
If you don't mind bending the rails it works fine. Otherwise you may want to look at other designs that don't require modifications.
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