For the most part, my running career has been as a soloist. I don't have a huge circle of friends and none of them run, so I've done most of it by myself.
It hasn't been for a lack of trying to find someone who would train with me. Once or twice I found a few adventurous souls who seemed gung-ho about it at first, but then for one reason or another just sort of drifted away after a while. It's been a source of frustration, at least up until now. I met Donna at the Ho Ho 5K in December, and we hit it off immediately and decided to start training together, since we share a fairly similar pace. She and I have committed to train all the way to the marathon, which is one of the reasons why I feel as positive as I do about my chances of success this time around.
Of course, running solo does have its merits. For instance, I tend to do a lot of my best thinking while running, at least after I'm no longer consumed with those first fifteen minutes of settling into pace. Being alone with your thoughts is sometimes preferable, and it's hard to stay in your own head when you have a companion. Running by yourself also means you don't have to keep up with anyone but you. You set your own pace, keep your own pace as you desire, and finish whenever you want. You have your own sense of accomplishment when you've finished since you don't have to compare your efforts with someone else's.
I think, however, that having a training partner is more beneficial than not. Having someone to help push you (or drag, as the case may be) can be quite helpful - perhaps you decide you can go just a bit longer and/or harder because you know the person with you will do so - and from that extra effort, you further develop your own skills. Being able to chat with someone is also very nice - it can help keep your mind from caving in to the negativity that creeps in when it gets difficult, like when you're staring down a nasty-looking hill on your route. But mostly it's just being with someone who is sharing your experience.
I have found that when I run with other people, the comraderie is there regardless of how well I know the other person - it's always been easy for me to settle in next to a total stranger at a race and just go. We both know what the goal is, and falling into a common pace helps set the tone where we don't need to speak unless we want to. It is often sufficient just to run side by side, like an old married couple who learned early on they don't need to fill in every silent space with words, and the knowledge that I'm not alone is enough.