I've owned both and think I'm (more or less) well acquainted with the sounds of both so here's a quick review comparing and contrasting the two pedals. Both are excellent and excel at different things but are very good at what they were designed to do. Overall, the FL5 is designed to be more of a classic rock tone machine while the RK5 is designed to be more of a hard rock/lead players platform. First off, the FL5: Initially I didn't like it. I was expecting a Fender clean and Eric Johnson type lead tones to immediately jump out of this thing. What I got instead was indeed a Fender clean type of tone but the Plexi circuit just didn't jive with me. Playing alone in our basement, when I engaged the Plexi circuit the mids were brought to the forefront and the tone got fatter with a little grind and my guitars took on a "hollow" sound that I wasn't nuts about. The boost added some grind and volume but I'm a lead player and I was looking for more. The delay and the reverb sounded great and were an outstanding match for the pedal. Playing the FL5 live: Holy crap this thing sounded great live! All the "hollowness" I heard when I was playing alone in the bedroom translated to great rhythm tone live. I'm mean- GREAT tone live. The crunch was just right for virtually every song and the tap tempo function on the delay made it priceless for spacy rhythms. I didn't feel a great need to even use it without the Plexi circuit engaged because it had such a great feel with it engaged. the boost function allowed me to get some good lead tones but I preferred a separate TS pedal for better lead tones. The RK5: If the FL5 is of the Marshall flavor the RK5 is in the Mesa camp. The gain is very very similar to my Mark III and the distorted tone is a hard rock player, or shredder's dream. As near as I can tell the clean tone, reverb and delay are very similar if not exactly the same as the ones one the FL5 so the primary difference is how the OMG (versus the "Plexi" on the FL5) option interacts with the rest of the circuit. I have had a blast with the pedal. I've used it with my Les Paul, Strat and Peavey HP Special CT and it sound's amazing. I'm primarily a lead player and it has a tight, punchy tone that really cuts through nicely. The OMG section offers a very aggressive tone but dialing back the gain and tone yields some nice grind for rhythm playing but it's forte is searing lead playing (IMO). It does react very well to my guitar's volume knob but for cleaner parts I still need to turn off the OMG section which is totally fine with me. Overall: I haven't gigged with the RK5 yet (I will tomorrow night) but it is the more fun of the two for me and my fits a little better with my personal style. I can't enough good things about how the FL5 sits in a band mix however. Everything I didn't like about it alone I loved about it live. I hope the RK5 will be similar. We'll see soon... Let me know if you have any questions or comments. All the best!