Here’s the downside and my biggest gripe: distortion

The distortion of the lens is so great that landscapes are essentially impossible.

The Rokinon is dead sharp at f/2.8 on the XE-1, and its well-known mustache distortion can be corrected with Photo Ninja's distortion tools

It does however have significant "mustache" distortion

The lens profile was easy to install for lightroom and corrected any distortion though my crop sensor takes care of most of it.

I did need to hunt down a lens profile for LR to correct the moustache distortion.

But this one provides excellent wide angle abilities with out an intense distortion... especially on a DX.

Plenty sharp, distortion is not terrible for a 14mm, and I always just leave it without correction

As far as perspective distortion, well, that's up to you

I highly recommend both this lens, and PTLens software to correct the distortion.

Once you do, majority of the distortion will disappear and you can correct any other distortion/vertical/horizontal manually.

It produces complex distortion which is a combination of barrel and pin cushion distortion

If you do, then distortion is easily corrected with a current lens profile from Adobe's Lens Profile Downloader (if you're an Adobe Camera Raw or Lightroom user)

Plenty sharp, distortion is not terrible for a 14mm, and I always just leave it without correction

If it's bothersome, Adobe camera RAW does have a decent profile that corrects the distortion pretty well

I swapped mine with a Nikon cap.-Has significant mustache-type barrel distortion in the middle of the frame.

I decided to return to an earlier idea of using this adapter with the (potentially...) very sharp Samyang/Rokinon 14mm f2.8 lens for a 28mm-equivalent PC lens, even though its linear distortion is quite severe (as is sample-variation with this manufacturer, and it took three tries to get a good sample of this 14mm)

I may try using DXO Mark's software to see if it can correct the distortions better

Another drawback is the distortion, but you can fix it with adobe lens profile or PT lens easily.

While I'm no professional architectural photographer, I corrected the distortion as best as I could in Lightroom and came out surprisingly straight.

The only downside is the complex distortion which I already knew going in, can't take one star away for the forehand knowledge.

The distortion in landscape shots is invisible to me

A strong value and well made lens at the ultra-wide end, given it's manual focus as well as mild distortion constraints, one thing the Rokinon can do very well is time-lapse: capture sunsets, moonlight, starlight, clouds, and great colors along the horizon, leveraging the AE-chip that will adjust metering according to changing light conditions.

It has virtually no coma, far sharper corners than any comparable lens outside the aforementioned Nikon 14-24, and for landscapes the strong lens distortion is quite correctable in post-production

Mustache distortion easily corrected with PT lens.