I ride both flat bar and drop bar road bikes. I prefer the flat bar riding position (more upright) when I am tooling around the city or the bike paths, it is more comfortable to keep my head up. I regularly ride my flat bar bikes on 40-50 miles rides and I've done centuries on both of the flat bar bikes, no problems. If you like the riding position of a flat bar, Specialized, Cannondale and Giant all make great flat bar road bikes. My brother has a Trek FX 7.6 and it seems to lean more towards mtb geometry and gearing imho. My Quick 1 has a 10 spd Ultegra with a compact crank and it flies.

Biggest negative I've found riding the flat bars is difficulty keeping up in a group ride when they are really hammering and there is a stiff headwind (15+ mph).

Before purchasing my first drop bar road bike I also considered converting one to a flat bar. REALLY glad I didn't do that, I love riding my Madone and CAAD. Had no problems getting used to the drop bars and more aggressive riding position. Now I just have to remember how to shift....SRAM on the Madone, Dura Ace on the CAAD.

Just my 2 cents.