I've played with the trial of ACID (latest version is 5.0b, BTW, not 5.1). I also bought a couple of "country loops" CDs because I don't have a pedal steel. But I don't use acid because I don't use loops for most of the music or even the rhythm "bed" tracks. Just a few thrown in for flavor 'n spice. So I just use the loops in Vegas (multitrack software).
If you're going to actually build tracks primarily with loops, ACID is very well respected. If not, just use multitrack software (Adobe Audition is another good one, Audacity works and is free, and a thunderin' herd of Protools people will come out of the woodwork to tell you their choice is the professional one ;-)