A friend invited a few of us to her country place for a retreat a while back; it was informal (as in, not an 'organized event'), but we set up strict rules and it worked really well. We had set break times for chatting and relaxing, and did writing prompts after lunch and in the morning to get the words flowing. At night, we read from our WIPs and critiqued each other's work.
It was really useful and I got a lot of stuff done. I think the main thing in an informal sort of retreat like this (whether it's with others or on your own) is to set firm ground rules and goals.
I went on another retreat a month ago, but this one was really more of a conference, with workshops etc; you could slip off to write if you wanted to - you weren't obliged to stay at the event (we had lovely cabins), but I was there to learn and to network, so didn't take advantage of that.
Also, in January my local group organized a one-day mini retreat; people could drop in during the day for as long as they wanted and write, and then in the evening we split into two groups, one workshopping query letters, the other doing a general q&a session. Again, we had ground rules in place for the daytime portion: one room was designated as a 'chit-chat room' for anyone who wanted to talk, and writers were asked to respect people who were working and keep the noise down.
So yeah, retreats can take many forms, but I got a lot out of all of them.
My best story. I was booked to go down for a convention to Dublin, on a one day turn around since hotel prices are so bloody expensive down there. So that's the train at 6.30 and home that evening about 10.30, running across Dublin (a city even the locals get turned about in), doing two panels, meeting everyone and going home. I was already very stressed after a busy week (some of you will know I have a low tolerance for too much stress) and I took a panic attack on the train on the way to Dublin. Which meant I got there exhausted and tired and very wired up.
Anyway, I got to the con, knew I was in no position to stay for the late train and arranged for the very gallant Allen Stroud to cover my Space Opera panel at 5 and planned to go for the early train. I did my one panel, did well, thought things were back on track, but when I went to catch the bus they wouldn't take a ten euro note. I ran to the Tescos across the road, and missed the next bus whilst they changed my money, and the next one was going to make me late for the train that I'd rearranged everything to catch, and I was feeling weepy and shaky and very sorry for myself.
Anyhow, being an active heroine in this story, I decided to use the ten euros and get a taxi (Dublin taxis are also atrociously expensive) to the train station. And then the taxi driver asked me how my day had been, and he was lovely, and I was on the point of crying, and he must have realised because he started distracting me by asking about the convention etc, and I must have told him I was a writer.
At which point, he handed me a business card for a cottage in the wilds of Donegal, lovingly restored, without wifi etc that he told me I could have for free any weekend over the winter to write, if I wanted.
I never went (Donegal in the winter redefines windy, wet and freezing) - but the thought was so kind, and he put me back on my feet and I got home just fine.