The biggest pro I can think of is that this excellent software is open source, meaning it's developed from a community driven perspective i.e. users have a voice and can develop and add features as they see fit.It supports a wide variety of platforms, has a GUI and CLI interface, and supports the a pcap variation on every one of its platforms.It's filter creation tool is top notch, letting you specify what traffic you want to see and how many packets you want to see.You can actually export packets to text files for later review if need be as well.
Room for Improvement:
As some of the other reviewers here have stated, one con is that this software is only an observer, not an interactive component of the network, meaning you cant change anything with it.It also lacks a few modules that other, closed source software's have, but I have no doubt that the community will come up with a solution soon for that issue! It's continuously being developed and changed.
I've been using Wireshark for a long time, since back in the days when it was still called Ethereal. Since then, there has been no alternative for me for Packet Sniffing. Wireshark does exactly what I want and leaves me with no need to look elsewhere.