Other than beach crusiers and BMXs, I bought my first singlespeed mtb about 10 years ago when my girlfriend started riding so she could keep up with me. My god it was the best riding decision I have ever made, I absolutely love riding them now and only own one geared bike which is for sale as I never ride it. (Bought it last year since everyone gives me crap for riding SS).
I cannot explain why I like it so much, the simplicity, not even having to think about gears – crunches and slips, a directness you don't get with gears. For me it's just awesome, I never seem to be behind the geared riders and I even happily did the Southdowns in 2 days last weekend on one no worries.
Yes I ride with discs and suspension.
The one and only time I have struggled was a 120 miles one day trip on a SS road bike.
I am very curious to try a internal hub though.