As I rule I try and avoid them. Mostly a pain in the a**e!
They are two completely separate lifestyles and I've been happy and less so in both.
I enjoy being single and not having to worry about anyone else. It's when I need to work late, or if that quick pint after work turns into a session that I don't need to let anyone know and risk disappointing them or spoiling plans. I like that I can do pretty much what I like, but then I've been in relationships where I haven't been constrained really anyway. Even then though I felt bad if I was going to be doing things without my partner, especially if I knew it was something that she might enjoy.
The downside to being single is not having someone to share experiences with, not being able to play a part in someone's life, or have them in yours. Simple little things that happen daily that you think to yourself "damn, I wish x was here to see that".