I'll more or less echo the above.
For T4, AFAICT all of the products were at least slightly marred by the incompetence of Imperium Games; some are still worth having, most aren't.
Book by Book:
1. Rulebook: no need to get this; it's got more errata than MegaTraveller and the rules (where they're changed from The Traveller Book) aren't very good. The map of Core subsector matches neither previous CT canon OR the later Milieu 0 books.
2. Starships: worthless. The ship designs don't work under the rules, the deckplans are worthless (and hideous!), the SSDS is badly organized (and IIRC available for free elsewhere on the 'net)
3. Central Supply Catalog: pretty much worthless. The collection of non-weapon equipment is servicable if uninspired, the vehicle design system would be OK except that it's totally incompatible with everything else in Traveller...
4. Aliens Archive: possibly worthwhile. A collection of 10 new Minor Alien Races. Thery're not IMO up to the level of the old CT-era races but if you like new aliens you might get some use from this book. Fun little trivia note: all T4 books were 112pp long; this one apparently didn't have quite enough content so to fill pages they MADE THE PRINT BIGGER.
5. Milieu 0: probably worthwhile. Details of the 'dawn of the Third Imperium' setting. Horribly organized and edited with absolutely wretched artwork, but there are some gems (and some clunkers) buried in there. If you're interested in playing in this setting (or just Imperial History in general) you'll want this book (though see below).
5a. Milieu 0 Campaign: Books 5 & 6 bound together into a single hardcover volume with 32pp of new material. Since Book 6 was absolutely worthless the choice of whether to get this version or the softback version depends on price difference vs 32pp of extra material.
6. First Survey: painfully worthless. Starmaps and UWPs for 10 sectors. Unfortunately, EVERY ONE of the UWPs in broken -- govt. code and law level are identical for every single world. Even if you fix this problem (and IIRC there's a fixed version online somewhere) it's STILL worthless because the numbers are all random with no concern given to common sense or prior canon (for instance, Vland itself is listed with a population of 10,000). Adding further insult to injury, the whole thing is printed in a space-wasting Courier font so that maybe 40pp of material is spread over 112pp.
7. Emperor's Arsenal: possibly worthwhile. A catalog of weapons organized by tech level. Some good descriptions and lots of very good artwork by Bryan Gibson (this was the first book after IG canned their original design staff). Unfortunately the weapon stats are all in T4 format so unless you're actually playing T4 usefulness is limited.
8. Pocket Empires: Best book of the T4 line. Rules for creating pocket empires, including large-scale economics. Can also be played as a large-scale strategic game.
9. Anomalies: worthless. A collection of adventures, most of which are very bad.
A. Psionic Institutes: possibly worthwhile. Everything you ever wanted to know about Psi Institutes, including adventures. Very solid book, but of limited usefulness unless your game is set pre-Psionic Supressions.
B. Fire, Fusion & Steel: gearhead craft and weapon design, an update of the TNE book of the same name. Gearheads seem to like this book, but it was so mangled by IG as to be almost completely incomprehensible (for instance, they mangled the format of EVERY SINGLE EQUATION in the book). Errata is available online somewhere.
C. Emperor's Vehicles: worthless. Nice art by Bryan Gibson, but the vehicle stats are missing so much crucial information (for instance, tech level) as to make them useless for actual play.
D. Imperial Squadrons: probably worthwhile. Info on running active-duty naval campaigns, and rules for strategic-level engagements adapted from Fifth Frontier War.
E. Naval Architect's Handbook: collection of deckplan modules to cut and paste together. I don't have this book but I understand it's worthless (plans aren't all the same scale and/or are stupid/canon-breaking).
F. Nobles: never published; there is no F
G. Missions of State: collection of noble-themed adventures that (per my understanding, I don't own this) weren't very good.
Long Way Home & Gateway: two-part adventure, originally published by BITS in 1 volume. Not bad adventures, if you can stomach the really cheesy (and canon-breaking) 'interstellar transporter' McGuffin.
The Annillikk Run: worst Traveller product ever? Absolutely dreadful (and canon-ignoring) D&D dungeon crawl in space.
GT, by comparison, is absolute gold. Although some GURPSiness tends to creep in around the edges, the books are generally rules-light enough that typicallly ~80% of the page count is useful even if you don't use the GURPS rules. I won't do a book by book rundown, but suffice to say that depending on your area of interest (and what you can find -- several of the earlier books are now out of print) pretty much all the GT books are worth picking up. My personal favorites: Far Trader (applies RL economic logic to Traveller for the first time ever), Rim of Fire (guide to Solomani Rim sector; more info than we've ever previously had about this region), and Planetary Survey 1: Kamsii (a single world, exhaustively detailed -- in this case a tourist/resort planet in Core sector). First In (Scouts/WBH upgraded), Starports (just what the title says), and Ground Forces (everything you ever wanted to know about the Imperial Army & Marines) are also very popular among the fans.