If you are going to be desert camping where nights are going to be in the 50's or so it may be too warm, however i find that i can usually just zip the bag open and use it more as a blanket and works great.
If you are going to be doing high elevation camping then it will probably work great for you.

Judging by where you live and for the 3 seasons you'll be using your bag, it sounds like a 30 degree bag is a good temperature rating for you.

If you are set on Big Agnes sleeping bags, check out these 30 degree BA sleeping bags we carry:

http://www.backcountry.com/store/BAG0097/Big-Agnes-Crystal-Sleeping-Bag-30-Degree-PrimaLoft-Down.html

http://www.backcountry.com/store/BAG0093/Big-Agnes-Fish-Hawk-Sleeping-Bag-30-Degree-Down.html

I personally prefer down to synthetic bags... you pay a little more, but they are generally a bit lighter and you can stuff them to a smaller size. I also like the feel of them better. If I had to make a recommendation outside the line of Big Agnes sleeping bags, I love my Mont-Bell sleeping bag to death, and recommend Mont-Bell products:

http://www.backcountry.com/store/MTB0047/MontBell-UL-Super-Stretch-Hugger-3-Sleeping-Bag-30-Degree-Down.html

http://www.backcountry.com/store/MTB0088/MontBell-Super-Stretch-Burrow-3-Sleeping-Bag-32-Degree-Synthetic.html

The first bag is pricey, but is worth every penny. The weight of this bag is next to nothing, and it stuffs down to a very small size. Great for backpacking. I have this bag in the 35 degree and love it to death. The second bag is a synthetic 32 degree bag. Both of these bags feature the stretch feature, which is nice for those slide sleepers who would like a little more room in their bag.

Hope this helps - let me know if you got any other questions!

I live about 45 minutes from Daniel Boone National Forest in eastern KY (aka Red River Gorge/Natural Bridge State Park). The weather is pretty similar to the smokies. I'm mostly interested in a bag, preferably Big Agnes, that I can use for spring/summer/fall camping/backpacking. I don't intend to do much if it gets below 30 degrees. Do you have any other recommendations?

It honestly depends on where you are planning on using it. Hiking in the Smoky Mtns, it may be a little too warm, but hiking on the Colorado Trail? Maybe a little too cold.

If you happen to be hiking where you think the low temperatures won't get below 30 degrees, it would be fine, but it could be too hot. Where are you planning on going?

I'll tweak both answers and add that what matters most isn't the length of the zippers, but that they're the same type of zipper. As long as they're the same type (YKK #8 for example) and mesh properly it actually doesn't matter if one is longer than the other. As an example I have the Cat's Meow Long (TNF) and my wife obviously has the women's version and though its zipper is shorter it just means the zipping ends where her zipper ends--my zipper could keep going further up but doesn't. It's not a problem, just a limitation.

As long as the zippers are the same length and are right and left zippers, and same type zipper.
This Gun Creek bag will work with most of Big Agnes' other bags.

Yes and no. Yes, BA bags will zip together, regardless of shape or model, if they have the same size zipper and one bag has a left zipper and the other bag has a right zipper. No, this bag (70" zipper) will not zip together with the Horse Thief (60" zipper) since the zippers are not the same size.

You can go to http://www.bigagnes.com and look at the specs for the sleeping bag(s) you are considering. The description of each bag has the zipper size listed.

All you need is to buy two bags of from the same brand but one bag needs to be a left zip and one bag needs to be right zip. Long or regular won't matter when zipping together.As long as both zippers are compatible, they will zip together.