No, she is not a good friend. Mostly because Buffy is incapable of separating her private life from her calling. Her friends become automatically and by default a part of the slaying gig. Buffy does not treat them as equals, she treats them as minions, choosing to either act as friend or as a slayer when she feels like, lying to them and keeping secrets in every season up till the end.
Yeah Buff, we get it, they are not slayers.
No, really. They do not wield the strength and skill to stand against the forces of darkness, whatever power they have they claw it on their own and pay for it. Buffy doesn't understand that. It's a problem because Buffy doesn't treat slaying as seriously as she should and her friends suffer because of it, Giles, Willow and Xander lose their loved ones because of her incompetence as the slayer. She fails to protect them. On the top of that, as the seasons progress Buffy becomes dependent of not only their companionship, but their power as well, to the point where Willow has to deal with a big bad on her own because Buffy refuses to.
Take a look at Buffy and Willow. Buffy knows that magic is demonic in nature since the second episode, she knows what magic does to and with humans since the third one and yet she expects Willow to be her big gun and asks her to give more and more with every season, consequences be damned. It's incredibly toxic. When Willow finally falls apart in season 6 Buffy barely cares. In the final TV season Willow is doing worse than ever, she casts powerful spells involuntary, loses control of herself, sucks energy out of people and who knows what would have happened in "Get It Done" if Xander had not intervened. Buffy still asks to cast more and more powerful spells.
Would you want her as a friend?
Yeah, why not? She's a fun gal, loyal, huge heart but more importantly, she is a ridiculously interesting person. It's like with Willow, no matter how repulsive and rotten she is, she IS Willow Rosenberg, yes please, I want her as a friend. Of course, the fighting evil part is out of the question for obvious reasons. I kinda like bad skating movies though, "Ice Princess" was pretty entertaining.
Does her responsibilities, stress of slaying, etc excuse her bad behavior towards her friends, or make it more understandable?
No because slaying is not the reason why Buffy behaves in the way she does, she admits it herself in "Touched".
Is Buffy too self-involved?
Is the sun hot?
Are there any times people feel that she didn't mean well?
I think that she didn't mean well when she decided to kill Faith and feed her body to her vampire corpse-friend. Sickening. Or every time she lied to her friends about things that could potentially kill them. Or when she wanted to leave her kid sister under the protection of a soulless vampire that tried to rape not a while ago. Or when she asked her best friend to incapacitate and kidnap said sister instead of just talking to her(!) Or when she refused to slay a soulless monster, valuing her feelings over the lives of humans she was created to protect. Or when she selfishly chose the life of one person over the lives of people she could save had she not killed herself. Or when she decided to completely ruin the lives of thousands of girls and women around the world.
Off the top of my head.