I'm an awful lot like you, with the very few people I've let come close to me.
I do find that a lot of people who aren't all that close still come to me with their problems. I enjoy puzzles, and I love figuring out solutions to problems, so I'm a good sounding board. I don't gossip, and I don't get emotional, which probably makes me pretty easy to go to with issues, I dunno. Sometimes it's irksome, but I try to limit contact with most people so I'm not overwhelmed.
I will say that I've dropped out of peoples' lives when shit got too heavy for me. Just fell off the face of the earth, no explanation, no nothing. I've done it a few times, and I'm not terribly proud. I'd say this is my worst quality. One friend was just so emotionally needy - she'd been a close friend in highschool, and her life was COMPLETELY fucked up in adulthood - drugs, affairs, all sorts of INSANE drama. I tried to be her (phone) friend for a few months, and it was so draining. I'd need weeks to recover from one conversation. Finally I stopped responding altogether.
There were a few others like that, just leaning on me SO MUCH, and I couldn't quite access the right way to handle them. I try to keep a pretty safe distance between myself and other people in general, but when they are too needy, it's claustrophobic. And I lose interest, fast. I still feel some guilt over them - but I've let so much time pass, there's no way to mend the fences without a confrontation. And it would be an emotional confrontation, which - SHUDDER - I can't take, at all. I figure, with friends like me - right? If I dropped them as a friend when they needed me most, they probably don't want me as a friend when their lives are going well.
It's a shitty way to be. I don't like it. Just did it to someone recently, and I'm still kicking myself for being so closed off and self-protective.
So, I'd say YES I'm a good friend in general - but I keep a bit of a guard up for a reason, and when you continually break down my barriers I'm probably gonna run.