I'm currently off the island, and having a pleasant time at a rather badly organised conference. The usual reactions when it comes up in conversation where I'm from; bemusement, followed by some variation "It must rain there a lot".
The first winter we were here, we had frequent showers, hail, and wind for days on end. Last summer was somewhat patchy.
However, last winter was surprisingly dry. And the weather this spring, going into autumn, has been excellent. Crofters are optimistic of good crops, while the machair isn't the collection of puddles that it was last year for some of the time.
Where does this misconception come from? Probably the TV; the last two programmes I have seen about the Outer Hebrides have focused on dire weather, or made some passing comment on this. Whereas the truth is a little different. Basically, the weather can be the same as the mainland UK, only a little more extreme in either direction. When we have a sunny spell, it can be baking hot (on our first trip, I was sunburnt while lying on the beach at Vatersay). But when it is windy, by heck it is windy. And Tiree, an Inner Hebridean island not a million miles from here, often takes the UK record for the most sunshine in a year.
What to expect when you visit? Whatever time of year, I recommend wet weather gear, but also swimwear and sunblock. Just don't bring an umbrella.