This is not really a loaded question but has so many facets... I'll limit myself to the major points
Pros:
Sniffer has, at present, the most scalable solution for folks who intend to use it on large networks where many analyzers are deployed. Most other systems with any kind of distributed capability have added it recently in an effort to displace Sniffer. Some are just using proxy software as an ad hoc remote solution (Timbuctu etc)
Filtering is fairly intuitive and Expert supposedly identifies problems more quickly than the Expert systems of their major competitors.
They are the only distributed analyzer company I know of who truly has solutions for just about every topology - Ethernet, Token Ring, Gig, ATM, DS3, WAN serial, WAN T1/E1.
The interface itself is fairly intuitive once you get accustomed to it and the same interface is common to all topologies - makes transferance of acquired skills easier.
Sniffer University classes are generally very good - I've taken four of five classes and all but one were excellent (the one that wasn't so great was because of an outdated curriculum that has since been updated).
Lest you think I'm a Sniffer cheerleader...
Cons:
Way too pricey.
NAI has a crappy attitude regarding resellers and does not make enough effort to listen to what customers really want.
Front line tech Support is so-so and back line is good if you get the right person but they have lost a couple key back line support engineers lately (one in particular who was golden and they should have made more effort to retain)
Their Gig solution's capture capabilities are only now starting to catch up to Finisar's (Shomiti) THG solution and the Finisar hardware is still superior.
They STILL don't have a good solution for Distributed full duplex 100mb over copper analysis - the current answer is a four port Ethernet Sniffer w/special drivers that bind pairs of monitor cards together so the ET05 works as a dual full duplex analyzer but it has limitations.
Tendency to rush products to market before they're really ready for release.
WAN products not really in the same class as some others.
All told, they're still worth considering if you have the money and most folks in big and busy networks wouldn't dream of being without Sniffer but there's plenty of room for improvement.
I have opinions about the alternatives (Finisar, Acterna, Network Instruments, Etherpeek NX, Fluke etc) but prefer to limit this online discussion to Sniffer.
Feel free to email me to discuss at greater length.
Please note that my employer does NOT resell any analyzer solutions nor do we have any official affiliation with any of the companies I've mentioned (but we have had product development discussions with all of them regarding peripheral products we manufacture).
Owen O'Neill
Datacom Systems Inc.
Northeastern SE