I've tried both TrafficX - at great length and toil - and Ultimate Traffic II. I've also used the original Ultimate Traffic on fs9 for years.
I have to say the UT2 is a far better product in many ways than TrafficX. For one, it uses real-world schedules. Now, naysayers would argue that those schedules are only for one week, and real-world schedules tend to be more dynamic over a period of time. Well, I live within sight of San Francisco International Airport (KSFO), and when I see specific flights and aircraft types outside my window being mimicked by the exact same flights and aircraft types in FSX within a few minutes or less of each other - especially with international flights - then I think it's well worth mentioning that the results with Ultimate Traffic II are impressively close! Personally, I was a bit put off by the results of the 'iffy' nature of the fictional schedules generated by TrafficX, which, by the way, also produced a lot of undesired results, which I will get into in more detail later.
Secondly, UTII works as advertised 'out of the box'. This was a huge issue I was having with TrafficX, which uses an interface called the Traffic Control Centre, or TCC. It is well laid out, has many potentially useful features, and is fairly simple and straightforward to use. However, as I discovered over and over again over multiple hours of tinkering, the TCC does not nesessarily produce the results you selected/deselected and compiled, and the appearance of many flights at certain locations in FSX that were generated by the TCC didn't make sense.
For example, one of the major issues I struggled with was that the TCC kept producing numerous local European commuter flights in the USA (e.g., Hapag-Lloyd flights flying out of KSFO), and vice versa (e.g., SkyWest flying out of EDDF). This occured at every single major airport I visted in FSX, and quite frankly, it was really getting on my nerves! Now, the TCC does have an option to disable or limit certain flights and compile your preferences, which I did numerous times, to absolutely no avail. At one point I even went through each flight individually and selected/deselected the ones I wanted to appear. I also checked, double checked, and edited as nessesary the respective flight plans. As one could imagine, that took a considerable amount of time over several days. When that didn't work, I resorted to completely disabling commuter flights altogether worldwide - or I though I did. Nope. Still kept happening.
I don't know about you, but I don't like spending money on a product that pretty much obliges me to go in and spend countless hours trying to fix something that really shouldn't be occurring in the first place.
Other issues was that certain types of B737s weren't showing up for certain airlines that use them, or older, unused liveries were applied to aircraft even though I selected a timespan of 2008-2009 within the TCC. I would still get the old-style blue, red, orange, and white United scheme on the old-school 737s with the slender, noisy turbojet engines they used to be equipped with, showing up at airports, again with only 2008-09 selected in the TCC.
Thirdly, many of the aircraft models themselves in TrafficX were a bit below par in terms of their appearances. Particularly with the textures, which were often very blurry and washed-out. Especially the textures for 'Merican' airlines - what was supposed to be American Airlines. I'm not trying to sound nasty or anything, but the 'Merican' textures were easily some of the ugliest I've ever seen on an aircraft in flight simulation. Look at the splendid American Airlines textures in UT2 by comparison and you'll see what I mean...
In fairness, however, many other models in TrafficX looked great, the Lufthansa ATRs I spotted at Edmonton, Canada (CYEG) being good examples. No, Lufthansa ATRs should not be spotted in west Canada - even with fictional schedules - but they did look good!
JustFlight got many forums support complaints from myself and many others, and they did listen and reply, it's just that their solutions simply didn't fix the problems. They released a series a .pdfs explaining in detail how to use the TCC, but they only detailed some of the already self-explanitory aspects of TCC use and funtionality. The actual problems themselves were not addressed, and this was coming from a developer that built a solid reputation for their customer support. This went on for weeks, with no real solution ever being offered, just a pervasive 'it's not our fault' metality coming from their responses. To say I was disappointed is an understatement.
I didn't have long to wait before Ultimate Traffic II was released, though, and when it did I promptly removed TrafficX and installed UT2. My expectations were high from using Ultimate Traffic in fs9 for so long, and I was very happy with that. UT2 actually surpassed my expectations! The schedules were great, and most of the aircraft models look absolutely fantastic! Granted, the CRJs and A380s need some work, virtually the rest of them look almost as good as user-flyable payware aircraft. A neat feature is that the commercial AI traffic fly flight plans using the airways and intersections, and not the old style great circle routes as used to be the case with the original Ultimate Traffic.
The tools in the UT2 interface are handy and useful, and the Traffic view is fun to use as well!
Plus, there's nigh any hit on performance in FSX when using UT2, something I couldn't say about TrafficX, who's algorithm placed tons of traffic at every single airport in the FSX world - something you just don't see in the real world, and who's TCC functionality couldn't be relied upon to manually adjust that problem.
So, I hope I wasn't too windy in this assessment, but UT2 is easily the best product of it's type available. 'My Traffic X' might pose a threat, but JustFlight definitely dropped the ball with TrafficX.