The kind that goes inside the main triangle- example. I know some here have them. What are your pros and cons?

I have two different sized of the Nashbar model. I like mine. I like being able to store stuff is space otherwise unused. But it does render my second bottle braze on useless. It also would get in the way of some frame pumps. They're obviously not very wide either, so bigger objects don't fit very well.

It's a pretty nice place for levers, tube, multi tool, keys and such.

Do you think the larger nashbar frame bag could carry your repair kit, keys and cellphone and U-lock, plus a few groceries (2-3 cans, carton of milk, loaf of bread)? And are you able to keep your first bottle mount with it on?

Goodness no.

I don't have a bottle mount on that set of braze ons, but if I did, I don't think it would be usable.

As far as the contents mentioned, not even close. Bread would not fit. Carton of milk would not fit, unless you left it hanging out the side unzipped and then it would be in the way. U lock would not fit, I don't believe, unless it's a mini. A standard might fit, but not a particularly large one. The other stuff would fit, and probably a small pump, or co2 inflator.

It's really not big enough for any kind of errands, just your usual bike stuff plus keys, phone, like that.

It's nice because it fits into main triangle of almost all bikes

The Nashbar one's won't, but there is a relatively new product that might fit your wants/needs. It is looked at here.
Current model (designed for mtb use) holds two 50 oz hydration bladders.

I have a small one that holds the controller and battery for my DLG. When I do my Dummy build, I am planning to use a special frame bag that they make.

I have the Nashbar one, but I hardly use it as it blocks access both bottle cages. I want to make my own, small one that will fit in the front of the frame.

you've opened a whole new can of worms...

frame bags can be some kind of wonderful...

this is a set from Jeff Boatman at Carousel Designs that I'm going to use for the 2010 Tour Divide,

The problem with the ones like the Nashbar that mount against the seatpost is that they can only be so wide without interfering with pedaling. The shape doesn't lend itself to easy packing either.

The Nashbar one in particular (if it's the same one I've got) doesn't seem like it would last too long either. It's not good for much more than tools and tubes as others have said. A small saddle bag works just as well and doesn't block access to your water bottle cage.