Well, metal is more durable than plastic and Steelbooks fit more uniformly on a shelf. The quality of the artwork is obviously dependent on what you buy, but the metal does mean areas of the Steelbook can be embossed and therefore subtly three-dimensional.
I think they’re clearly preferable to a regular case with a bit of printed paper under a slip of plastic, but it’s personal opinion if they’re worth an extra £10–20 for a new one. Many do hold their value however. A normal Blu-ray will be half-price within a year, but a good/rare Steelbook could be 2–3 times more than you bought it for.