i weigh in on these a lot. i have different requirements for UWA lenses either than what other people USE or what they THINK about.
my previous UWA lens was a canon 10-22. this lens was AWESOME. sometimes i wish i still had my canon because of it... single tear rolls down my cheek...
now i've done a LOT of research on UWA lenses and here is my take:
first, RANGE is important. an 11-16mm lens is a wide angle lens only. a 10-24mm lens is a UWA lens, plus it zooms into wide-normal territory. anyone can tell you that the differences in FOV in the wide focal lengths are far greater than at longer focal lengths. 16mm compared to 24mm is a pretty big difference.
this makes the 10-24 a great lens that has many uses. i used to take my 10-22 to house parties. yes, i used it for "nightclub" style photogrpahy. 10mm was great for getting a whole room in the frame; 24mm was great for people shots with no distortion.
on the other hand, if you had an 11-16mm lens you would be stuck shooting only the UWA-wide shots. if you had an 18-55 you would lose the UWA completely. and i personally never carry more than 1 lens on me at a time.
this may be important to you, it may not. for me, a lens that can do TWO things well is far more valuable than a lens that does one thing very very well.
SECOND, i personally like to get in close with my UWAs. i love UWAs because they create dramatic scenes by exaggerating the spatial relationship between subjects. the have a unique perspective which is most apparent when you get CLOSE to your subject.
take a look at MFD and maximum magnification ratios of all the UWA lenses. the nikon 10-24 and sigma 10-20 f/4-5.6 (as well as canon 10-22) are the best in this department. all three of them have a .24m minimum focus distance with magnifications that range in 1:5 (nikon at 24mm) to i think 1:5.4 (sigma at 20mm).
believe it or not, i shot my 10-22 at MFD quite a bit. at a wide 20-24mm and .24m you have a unique perspective that not a lot of lenses can achieve. sure, the 18-105 also has a 1:5 magnification ratio, but you don't get that ratio until you hit 105mm. it's a completely different perspective.
whether at 10mm or 20+mm, being able to get close to my subject with a UWA is essential to me. this is why i have to skip the 8-16, 11-16, and even tokina 12-24 (which has a .30m MFD and 1:8 max mag.). the nikon and the sigma are the only two lenses which get close enough for my tastes.
of course i could LIVE with a .30m MFD... but given the choice? i want the lens that focuses the closest!
FINALLY, there's the whole f/2.8 factor. personally i don't see that as a huge benefit. i agree, that it will create unique photos. MFD at f/2.8, 16mm, i'll bet you can get some very interesting photos. but is it worth it?
i don't think it makes up for the other factors. f/2.8 won't benefit you in low light, as UWA lenses are already easy enough to hand hold at low shutter speeds. if you can't get 1/15s i don't think f/2.8 vs. f/3.5 is going to save you. second, DOF is already so great on a UWA lens that unles you are at MFD you won't see any noticeable effects.
this is why i've chosen the 10-24 as my UWA lens, as soon as i can afford it! may not be for a few months, but i can't wait. i miss my canon 10-22!!!
