Grad school at same undergrad university? Good or bad idea?

It doesn't look as great on a resume, but it's not terrible. I ended up doing that because the program I wanted was basically the only one quite like it in the country and just happened to be the at the same university I got my BA from. If what you're looking for exists with equal value at more than one university, though, it might be worthwhile to apply elsewhere; the advantage is that employers will see that you've done well at more than one institution, gotten along with more than one set of faculty. It also shows that you've moved around a little, which demonstrates independence. So if it's an option, great. But if not, again, it's not the end of the world.

It's probably better to branch out and go somewhere else. However, sometimes there are considerations that make going elsewhere difficult. I was married when I was in grad school, and the closest university was where I had earned my BA, so I applied there for grad school. I was offered a fellowship, so it worked out pretty well.

I don't think its that great of an idea because you wouldn't be able to experience a new campus, be around new people, learn from other professors and take in the knowledge from going to different school

Not great, since you're showing you know one place and one group of people's thinking only.

Might fail both.