Depends on the unusual.
Ducky and I have a very sexy unusual (https://steamcommunity.com/id/ReekSanchez/inventory/#440) the blizzardy One-Man-Army. So in that respect, unusuals can be great.
On the other hand, some unusuals are extremely expensive (like a scorching flames team captain), whilst I'm sure this is unbelievably cool, you could buy a phone at the same price... so in that respect, unusuals are overrated.
Overall, if you want a cheap and good looking unusual that's fine, but if you're someone who owns a lot of them or very expensive unusuals, ask yourself was it worth it?
I think they're cool but more overrated than underrated, then again it is basically Hat Fortress 2™, so maybe it's just fitting. Never had one, but I don't really care to have one either, as I don't really care about the cosmetics aspect of TF2 in general.