You could get the Nexus 5 looks and Experience by using Custom Roms but for that as gregbradley said you have to unlock your bootloader which means your WARRENTY IS VOID.
So be a bit careful before you decide to unlock your bootloader. You Want to get the NEXUS 5 looks then well use some launcher like Google Now Launcher, etc.
Still you wanna risk your phone's warrenty to get the same OS as that of the Nexus 5 then, well go on and unlock your bootloader.
There are many AOSP Roms out there in the Original Development Forum. Most of them has all the Nexus 5 OS features. CM 11 has just the same OS as Nexus 5. Other AOSP Roms, like SlimKat, Carbon, AOKP, PAC, etc. have their own features and pros and cons. So, Go around have a look, find which one would suit you, then decide on it.
Hope it helped.