I've tried the following monitors to pair with my iMac with Retina 5K Display:
1) Dell UP2414Q - good monitor, but being an early 4K monitor with MST, it has sleep/wake issues where only half the screen would turn on appropriately. Avoid.
2) Dell P2715Q - fine monitor, but I had flickering issues every couple of seconds. You can run 2nd display at 5120x2880 effective to match screen res of built-in display but things look a bit soft. Sadly avoid.
3) Dell P2415Q - excellent monitor, but not 100% Adobe RGB compared to UP2414Q. DPI is different compared to Retina iMac, so things appear larger when moved from one display to the next. Good value at $430 on Amazon.
4) Dell UP2715K - amazing monitor, will match pixel for pixel with what the iMac with Retina 5K displays, can do 100% AdobeRGB for color-accurate work if needed. Downside? It's $2000 which is a tough pill to swallow when a base Retina iMac with $2500.
My recommendation? If you care about matching screen sizes and are lucky enough not to have the flickering issue, Dell P2715Q will do. For those who want to have just another retina screen and aren't ready to spend $2000, P2415Q. If money is no object, or having matching pixel perfect screen res matters that much (keep in mind there is *no* other way to run dual 5K displays on a Mac, not even a Mac Pro), then the Dell UP2715K is for you! And while $2000 is a lot to swallow, it won't always be $2000, and it'll be a worthwhile investment.
HP also has a 5K monitor using a similar panel as the Dell for $1299, but as far as I can tell, thats only in very limited supply.