Only flown into Skiathos, never stayed there.
We've been to both Skopelos and Alonissos which are part of the Sporades, which Skiathos belongs to. They're more our kind islands - less boozy Brits, half way decent beaches and good walking, but we are very old, nearly in our 50's so are pretty close to being able to go on Saga hols.
I think Skiathos would be nice outside of July and August. The old town's quite nice to wander round for an hour or two and it's still a "real" Greek town which locals live and work in. If staying in the town, I'd avoid any waterfront accommodation which mostly looked run-down and would be noisy. The old harbour's a nice place to sip a frappe and watch the world go by. Don't know much about the resorts dotted along the coast. I'd guess they'd be great if they're right by your favourite beach but you could be anywhere.
If sandy beaches are your thing then Skiathos will be better for you than Alonissos or Skopelos - which tend to have pebbly beaches rather than sandy, though Stafilos on Skopelos was sandy and there's a main sandy beach on Alonissos too - who's name I forget. There's another small, secluded ( *taps nose ) sandy one off the beaten track on Alonissos, but I'd have to shoot you if I told you it's name.
Skopelos and Alonissos both involve a ferry / cat / hydrofoil transfer, which can be either a bonus or a hindrance. I think I posted some pics of Alonissos on the "Holiday Photo <something or other>" thread in the Upgraded Lounge ? Chico's posted some pix of his Cephallonia hol too.
Edit Hmmm, thought I'd posted some pics of Alonissos there but apparently not. Not uploaded them to Flickr yet but as they were only taken in June that's not too surprising. I'm almost as much use as Spike !!! ( Sorry lad, couldn't resist ).
All the Sporades are very green and beautiful but that greenery comes from water, or rain as you might call it, so, they're a bit like Corfu, more prone to rain out of the main season than say Rhodes / Kos / Kriti. We went to both Skopelos and Alonissos in early June ( different years ) and got wet a few times ... without going into the sea.
btw
Syros (
Some pix here )
and
Symi (
some other pix here )
are both boss, but again, you fly to one island ( Mykonos and Rodos respectively ) and are then transferred by ferry