the oneplus will (i imagine) have the limitations of cyanogenmod- no carrier video calls (using skype isn't the same) and no fm radio (like anyone cares). this was an acceptable tradeoff for me on the s2/s3 since i personally value customizability more.
one large factor in me preferring cm is that i'm supposed to be stuck on ics on my s3 since samsung didn't feel it necessary to release a jellybean update for my model. even using sammy based roms, i have to stick to jellybean 4.3 when i'd rather be on kitkat. with cm, you will always have the latest android flavour even if you have to wait a month for a stable release.
if you're worried about the storage, thats exactly why it ships in a 64GB format.
if you're worried about waterproofing- well i'm not going to wash my phone with soap like some of the more adventurous s5 users here. and my non-waterproof s2 and s3 have survived being dropped and immersed in water briefly several times. as long as you remove the battery and let it dry out properly, well at least i haven't had any problems.
the hardware on the oneplus is pretty top notch. probably the thing that intrigues me the most is the sony based camera. i always marveled at how 5mp cameras from sony and nokia were soo much better in quality and less grainy than my 8mp s3. the engadget review doesn't seem as promising about this feature as i'd hoped but i'll wait and see on release day if it truly is something you don't have to settle for with samsung.
yes it's cheap and it's a new company and this comes with potential risks. i'm going to wait a couple of months after release to see if there're any serious shortcomings like widespread sds or (non-replaceable) battery death. barring that, this is the first time i've wanted a phone in years, and i can certainly afford the s5 if i wanted it.
of course the oneplus one can be purchased with invite only.