My kids and I ventured out to the new Firehouse Subs in the Costco shopping center on the SW corner of William Cannon and MoPac a few days ago. What a delight! I won't count myself as a sub sandwich "expert" but the two hot subs we ordered were huge, loaded with quality ingredients, and bursting with wonderful flavor. Although warm, the bread was soft and I heard several comments around us about people liking a hot sub without the "tear-apart-the-roof-of-my-mouth" crunch of many hot subs that come with the bread also toasted.
The shop had a nice selection of sodas and both regular and sweet tea available - all self serve. Employees were friendly, outgoing, and very interested in making sure that our dining experience was pleasurable. For the kids, they offer up cute plastic fire hats. A big hit with my gang.
So if you're looking for a yummy sandwich (they do salads too!) or great to-go lunch in SW Austin, I'd highly recommend a trip to Firehouse.