IIDX can be really fun once you get over the wall that you encounter around the 4-5s. I find that the most fun charts are around 7-8s. 9s are challenging but still fun. Once you get to 10s+, it just seems more challenging and hard than sensible and enjoyable, imo. My peak was at the 9s because I always hit the wall that you get around 10s.
Turntable isn't a problem for me unless the song is super turntable heavy (I'm looking at you, good-cool songs).
Pop'n is super fun too like Mogsy says. The controller can get ridiculously loud sometimes though since you're essentially slapping giant hamburgers but the music selection on the Pop'n series compared to IIDX is vastly superior and varied. Last time I played Pop'n, I think I was doing somewhere around 24-28s? I don't play it as much though but I definitely could.
A real easy way that I personally find to be handy to reading IIDX and Pop'n charts is by first becoming acquainted to the colors of the notes on the screen. On IIDX, grey notes will be your bottom row of keys and blues will be your top row. On Pop'n, the colors are a little harder for me to get used to since there are 5 colors as opposed to IIDX's two, but they all go toward the center's. There is a special Pop'n note skin that helps if you're more accustomed to IIDX colors than Pop'n though and I found that that helps if you're color-dependent since it turns the notes from white-yellow-green-blue-red to just blue-tops and grey-bottoms. After a while of playing, you just get used to glancing at the color and position on screen and you improve that way.
Definitely high learning curves but once you get your technique down, the games are super fun. :3