It really depends upon you and what you shoot with. If you are shooting a 2x crop factor then it's a 30mm lens, focusing very handy. 1.5 a 22.5 not so critical. Shooting film, worthless.
That said there's another real advantage to the new model, the 52mm filters. For folks who shoot stuff with B&W and want to move colors around it is worth it's weight in gold several times over. Much handier than the ugly approach the old 15 had (I think required tape somewhere).
While the build looks much better, the older one is much smaller. The built in M mount is handy, but mine was only on a Bessa L when it was pulled out. Once or twice on an M but not often.
IMHO the filter size is the biggest plus. Build quality is a wash, nice but I do not think it's required. Size of the new OK but the old was teeny tiny, easy to stash in the corner of a bag. Price is another negative of the new, same optics and no finder.
I sold one some time back but do not remember the price. I really liked the ultrawide feel of it, a fun lens from time to time. For a while my kit was an M4-P, 15, 40 CVs with a 105 Nikkor and a Bessa L with a 25/4 CV stuck on.
B2 (;->