Commuter Bike Suggestions?

5 miles is cake on any bike.  I don't even think you need any racks or panniers.  A backpack is fine for the 25 minutes it would take you.   I think a lower end MTB like  you mentioned would do nicely.  Throw some city tires on it and you have a bulletproof commuter.  If you wanna hit the mountains one weekend, just swap back on the MTB tires and you are set for some dirty fun.   Only other thing you will probably need is some lights and some clip-on fenders if it is rainy.  

Option 2 would be a low end road bike from bikesdirect.com or similar. 

Get a hybrid.

My Trek 7.2 is comfortable, good on gravel and hopping curbs. It has a chain guard, so if you add fenders and wear a velcro strap around your pant leg, you can ride dressed for work, as long as you take it easy.

Days you want to extend your trip home, take some bike clothes and maybe use your road bike on those days.

I've run over glass twice and didn't flat. Commuting to work is not a time you want to deal with problems.

My brother buys $60 Wal-Mart bikes in NY because they get stolen so much.

I have a 24 mile round-trip commute and I ride a Cyclocross bike, which is perfect for the terrain.  My route is partial open road, some trails and lots of curb hopping.  The cross bike is sturdy enough to take the same abuse as my MTB but is much more comfy and faster.

05' Specialized TriCross

trek 7.2 here also. just under 6 miles round trip. yeh it's heavy but you get the miles in, and it is sturdy. i've flatted once in 2 years i cycle in glass central. plus i can take it down to the local water parks and ride there, which are either loose stones or mud tracks.

Agree 100%. A hard-tail MTB is perfect for the type of commute described by the OP.

Most mountain bikes are considerably heavier than any road bike.  But they are geared lower, so there is no issue climbing hills.   Most Mountain or hybrids have a triple ring, so you can run something like 24 chainring with a 30 cog on the back.   You'll be able to spin up any hill at 6 mph

I have been commuting year round for about 3.5 years now.  I have a Schwinn Voyager.  It cost me $250 and I still love it.  It is a hybrid.  I commute about 8 miles in each direction.  I would definitely suggest getting fenders, a front and back light (I use Cateye, they last 100 hours between battery changes), and some reflective gear.  I have an Illuminite jacket and tights.  Cars give me a lot of room because they can see me.  I also carry rain gear.   I carry a change of clothes in a $10 backpack that I think I bought at OfficeMax.   In the summer I run with road wheels.  In the fall I use hybrid wheels because the leaves and roads can get slick.  In the winter I put on the steel studded tires.   The weather beats the hell out of the bike so I would not use my tri bike. 

 The deal is to do it cheap, safe and be prepared for anything.  It is definitely worth doing.  I fill up my car about once or twice a month.