Just upgraded to 15.10. Booting up with nvidia driver directly gives glxgears running at expected result. Installing bumblebee and running optimus glxgears gives /var/log/system entry and no expected result.

As a note - I have a Dell XPS as well, and I get the same result with Ubuntu 15.04. My co-worker has the same laptop with Arch Linux and Bumblebee works for him with the same hardware.

Nov  8 22:56:48 localhost systemd[1]: Started Bumblebee C Daemon.
Nov  8 22:56:48 localhost kernel: [  231.533201] bbswitch: disabling discrete graphics
Nov  8 22:56:48 localhost kernel: [  231.533216] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:48 localhost bumblebeed[3235]: [  231.983606] [INFO]/usr/sbin/bumblebeed 3.2.1 started
Nov  8 22:56:53 localhost kernel: [  236.364242] bbswitch: enabling discrete graphics
Nov  8 22:56:53 localhost kernel: [  236.903991] nvidia: module license 'NVIDIA' taints kernel.
Nov  8 22:56:53 localhost kernel: [  236.903994] Disabling lock debugging due to kernel taint
Nov  8 22:56:53 localhost kernel: [  236.912955] [drm] Initialized nvidia-drm 0.0.0 20150116 for 0000:04:00.0 on minor 1
Nov  8 22:56:53 localhost kernel: [  236.912962] NVRM: loading NVIDIA UNIX x86_64 Kernel Module  352.41  Fri Aug 21 23:09:52 PDT 2015
Nov  8 22:56:53 localhost systemd-udevd[3260]: Process '/usr/bin/start-nvidia-persistenced' failed with exit code 127.
Nov  8 22:56:54 localhost kernel: [  237.457039] vgaarb: this pci device is not a vga device
Nov  8 22:56:54 localhost systemd-udevd[3268]: Process '/usr/bin/start-nvidia-persistenced' failed with exit code 127.
Nov  8 22:56:54 localhost kernel: [  237.462114] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.462168] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.462197] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.462223] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.462249] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.462275] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.462315] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.462341] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.478583] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:56:54 localhost kernel: [  237.821649] vgaarb: this pci device is not a vga device
Nov  8 22:56:54 localhost acpid: client connected from 3265[0:999]
Nov  8 22:56:54 localhost acpid: 1 client rule loaded
Nov  8 22:56:55 localhost bumblebeed[3235]: [  239.353495] [WARN][XORG] (WW) "glamoregl" will not be loaded unless you've specified it to be loaded elsewhere.
Nov  8 22:56:55 localhost bumblebeed[3235]: [  239.353549] [WARN][XORG] (WW) NVIDIA(0): Unable to get display device for DPI computation.
Nov  8 22:57:06 localhost kernel: [  249.219663] ACPI Warning: \_SB_.PCI0.RP05.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150619/nsarguments-95)
Nov  8 22:57:06 localhost acpid: client 3265[0:999] has disconnected
Nov  8 22:57:09 localhost bumblebeed[3235]: [  252.695710] [ERROR]Unloading nvidia driver timed out.
Nov  8 22:58:18 localhost bumblebeed[3235]: [  322.192337] [WARN]Received Terminated signal.
Nov  8 22:58:18 localhost systemd[1]: Stopping Bumblebee C Daemon...
Nov  8 22:58:18 localhost systemd[1]: Stopped Bumblebee C Daemon.