I've been using my Dell M6800 nearly daily for about 18 months; it has been a great performer - no problems at all. I purchased from Dell configured with an Intel i7-4940MX 3.10 Ghz / 4.0Ghz turbo CPU; 32Gb DDR3-1600 RAM; NVidia Quadro K4100M; 2 500Gb SSD Drives.
I use it at my desk with a docking station, and drive 2 Dell 27" 4k monitors at full 4k resolution, 60Hz.
I'm not a real geek, so I don't run any benchmarks, etc., but this laptop works fine for my uses: Adobe Photoshop, Lightroom, Premiere Pro, Audition, Media Encoder, the MS Office Suite, multiple browser sessions, Visual Studio (sometimes multiple versions running simultaneously for different projects); SQL Server, SQL Server Management Studio, dbForge mySQL Studio, IIS, Topaz Labs, WinPCap, WireShark, etc., etc.
I had one problem with the setup: it turns out that the Dell P2715Q monitors require a certain BIOS rev level in order for the NVidia card to run both of them full resolution at 60Hz. Dell engineers worked with me, and finally isolated the issue, and sent me two new monitors with the correct BIOS rev, and it's been fine since then.
The built-in display (upgraded to the full HD matte screen) is fine for normal work, excluding extensive photo and video editing. For coding, general office applications, browsing, it is fine. Battery life is good as well, I get 3-4 hours.
There were many options available when I ordered; essentially, I could pick nearly every component and configuration option I wanted. I replace an M6500 with this, and am happy with this upgrade.