As usual, the commenters here, hiding behind their screen name anonymity, are creating halacha standards. No one here is posek enough to make determinations, especially to create an issur that is not contained in our Shulchan Aruch or the recognized poskim. One may quote any of the “issurim” that have been stated by contemporary poskim, but the proclamations as appear in earlier comments are plainly irresponsible. They are no better than the medical advice shared by lay people waiting at the grocery checkout.
A smartphone is a device that has great and wide variety of capability. Some of these can be used for things that are clearly assur (no, not my statement, but the pasuk of ולא תתורו אחרי לבבכם ואחרי עיניכם). Inasmuch as the access to these temptations is effortless, the risk of falling into traps is great. That issue must be addressed. Halacha refers specifically to the prohibition of entering a מקום סכנה. Having stated that, there are many things that are Torahdig and praiseworthy that can be accomplished with a smartphone. If these are needed, then one can consider the ways to reduce and eliminate the risk. To rely on oneself to avoid the dangers is foolish. However, a properly filtered smartphone can be perfectly safe of the dangers.
The other feature that was alluded to by Shopping613 is the addictive aspect that can easily rob us of countless hours, even not schmutz. This is true for many things, not just smartphones, though devices seem to be quite adept at capturing attention.
And my personal experience with rabbonim is that many do have filtered smartphones. The reality is that many people, by nature of their activity and career, have a real need for the “kosher” features of a smartphone. I know of many individuals who posed their shailos about having a filtered smartphone to poskim, and were advised to do so. The blanket “issurim” that are proclaimed by non-poskim here and elsewhere are suspect, and more often than not, just overstatements without proper halachic support. I understand the negative side quite well. But that is not the universal halacha for all. Just check with TAG.