No multirom required for me. The install path was

Unlock+root Nexus 5-> install TWRP recovery -> Under TWRP install Cm11 + factory wipe + install 1.1.7.28 image on top of that -> upgraded to 1.1.9.28 with wiki instructions (OTA upgrade).
Later 1.1.9.30 and 2.0.0.10 as OTA.

I'm going to use it as my second daily driver and report findings here.

Problems so far (updated 2016.01.14):

Finally I gave up Sailfish with Nexus 5 because of call issues caused by stuck sensors (screen blanking didn't work during calls).

* Keyboard sound and haptic comes and goes. More often doesn't work than works. Same goes with keyboard sound btw. Less annoying if I just turn them off completely.
* Video recording crashes camera app
* No 4G
* Touch sensitivity is good, but it is harder to successfully trigger long press action compared to Jolla. Very noticeable in a moving bus.
* Calibration issue with brightness; suddenly went very dark and brightness slider run out of scale. Reboot fixed.
* Battery in the standby mode (~34 hours in idle standby). The problem is caused by sensor processes. See https://bugs.nemomobile.org/show_bug.cgi?id=837 . Battery usage during actual use of the phone is on par with Jolla phone.
* Because of sensor processes eventually getting stuck, you have to reboot the phone daily. This is to clear up situations where you answer the phone and the screen does not blank out but keeps getting contact with your ears and cheek.
* Some applications from Jolla store still don't scale properly (Compass as an example), but there are no scaling issues in OS itself anymore.
* Applications using HERE Maps as base don't work (Transit Live), as it does not exist on Nexus 5 (licensing I suppose)
* Scrolling in some apps (Contacts) is a lot worse than on Jolla for some reason. UX elements are very smooth, as is sailfish-browser too, so it isn't about the performance.
* Rotation stuck to landscape in applications once (no patches). Reboot fixed. This seems to happen once in every few days, and because of this it is best to enforce orientation into portrait under screen settings.
* Often calendar alarms cannot be acknowledged. You get the alarm sound but the screen stays off. When you open the screen, the alarm is not visible anymore.
* No account support for Microsoft's activesync. For calendar and contacts you have to use services that support CalDAV and CardDAV. Good to know beforehand if you are not in the position to choose from.

[* I claimed before that there was 'No word suggestions in virtual keyboard'. Actually there is; you have to install jolla-xt9 package. This was described in the first post of this thread. Stupid me.]

Obvious drawbacks:
* There is no aliendalvik, so no those few useful apps such as are needed for flight check-ins. It is this kind of thing is where it starts hurting if you don't have an extra mobile device with you.
* No double tap to open. (Kind of workaround is to have LPM mode enabled, so you can doubletap while the screen is on when you take the phone out of your pocket.)

Overall:
We all know Sailfish pluses such as swiping, good contacts app with xmpp-integration etc, so of course I'm very happy with SailfishOS on Nexus 5. (Update: Or was until the sensor problems started to be really irritating.)
It does not run out of memory, doesn't need swap, is stable and has a bigger and far clearer screen than Jolla has. It also has better pocketability, while being pretty much 1:1 in size. Thinner but couple of millimeters longer in the middle.

I'd switch to it as my main phone from Jolla anytime should it have aliendalvik. I like emulators on android, and they are almost nonexistent in pure Sailfish environment.