I've been writing for a long time and have written a considerable amount. Some have been published, some have been self-published and most are just gathering dust, or are locked away in my head somewhere. Probably way too much for my own good and more than I can write in a lifetime.
I prefer writing alone because I've often found that writing buddies - even ones who share your visions - will have their own ideas. Each contribution is valid, but you have an idea for a story, so you know its direction, its mythos, or its nuances in your head, so when someone else comes in and says, "Oh, how about this. Or changing this." it usually won't fit into what you'd first envisioned, even if they're excellent ideas.
I've had my share of writing buddies too. Most of them have been awesome. It's just a matter of learning to compromise, to adapt and to think how that cool idea is going to integrate into the storyline. Oh, and they point out spelling, grammatical or punctuational errors (I make very few of those), but also plot holes (which there tend to be quite a number of given that my mind races at 100 mph in a 30 mph zone!).
I've also had some really bad writing buddies. And I mean BAD!! One in particular (quite recent) actually had a very good idea for a story, one that would certainly work well in a game environment (which was going to be the intention). The problem is that the story closely resembled another game - almost too much so that we had to try and tweak things so they became unique. There were things in the storyline that just didn't make sense; they jumped from one thing to another and the readers/players were just expected to know why they were at that point.
The problem wasn't so much in all these flaws - those are inevitable in the first few drafts. It was that this person was not open-minded at all to any ideas but his own. Subsequently, we spent most of our time explaining why things didn't fit into the theme or work as a concept, or listening to him rant about how we were trying to take over HIS story and change it completely...Blah! Blah! And then there was the issue of copyrights (specifically the author's, artist's and game maker's materials)...I'm not even gonna touch that one! Haha!
Unfortunately, it got to the stage where we just had to abandon the whole project and wish him well.
I like working with others as writing buddies a lot of the time because they can give a fresh perspective on and old idea or shape parts of the story into something you didn't think of. My main writing buddy is - and always has been - my wife. She's a story-crafter in her own league, so when it comes to ideas she's brimming with them. She's also my inspiration and my Muse. :)
(I really hope)
