I have the old Electribe sampler (the green one), and it does alright, bit too much midrange, but through an EQ pedal and some decent reverb and compression it does fine for what I do.

My friend has the RS7000 and it's pretty awesome. I'm looking to upgrade soon and I'm wondering if I should throw down the extra $500 or so for the Yamaha or just get the new Korg.

There is never any frowning at Korg synth products. If it's one thing Korg is known for, it's taking care of the need. They are usually the firsts to come out with the answer to something. I love Korg for that, they have balls, big ones. Back in the days when Kurz was a youngster and just learning to shread cabbage for kraut (at synth's that is), someone may have said, "Hey wow man, who is your guitarist anyway?", and their eyes would get wide when you just said, "Oh, That's not a guitarist, it's a Korg."
I have not tried the ESX-1, but I am trying to stay away from the music store lately.