Well, as this is 1.5 we're talking about and I did Beta test 1.5 and know about the previous versions I think it'd be best to leave my opinion on M4-78.
First of all, Zybl revamped the intro to Korriban on the Hangar platform to use cut Atton lines from the E3 Demo instead of the spliced lines of 1.2 and before. While most of those splices where pretty bad *cough* HK-47 *cough* I felt some lines where remarkably good such as the one with Kreia where she says "Droids gather while we wait here" was absolutely well done it sounded completely natural, the only way someone could guess it was a splice was because the line "Our enemies gather while we wait here" is so iconic.
Most of them have been removed except for, you guessed it, the HK splices I hate. The Atton lines are alright though they do sound a bit awkward and Atton does say "you should keep your Lightsaber up or do whatever you Jedi do in these situations" despite the fact you may not have a Lightsaber.
In previous versions, the player would have to play as either Goto, T3 or HK through the radiation sometimes walking long distances to get to a location. To make the walking less like torture an optional sidequest was added in and was odd in the fact you got the quest from one foreman droid but depending on who your character was whether it be T3, HK or Goto this one Foreman droid would have an entirely different purpose, personality and sidequest with a different plot but same overall pacing. For HK this foreman droid was an off-world assassin droid hunting down a specific droid, for T3 it was a droid tracking down a computer virus intent on destroying the archon M4-78 and for Goto is was some sort of package retrieval mission.
At some point, you'd be stopped by three police droids who question your droid over the deaths of recent droids (2 droids or more depending if you did the optional sidequest) and they'd put your droids arrest on hold until they fixed the radiation leak. Once the Exile arrives on the planet the Exile can destroy the police droids once they threaten to arrest him/her for refusing to hand over the droids. All of this was removed in favour of a teleportation transport system to make the radiation section quick, the teleportation is, in my opinion, would be better if it showed your droid move away and then teleport to give that immersion that your droid did move away instead of being teleported.
While that filler sidequest is good for being removed I felt those Police droids should have stayed, Zybl2 actually forgot to remove the police droids 100% last time I played the mod which may have been the late Beta/first release of 1.5. Just before you fix the radiation backtrack a bit and you should be able to find the Police droids and approach them from behind.
On top of turning the industrial zone into 2 sections to reduce lag the actual research centre part has been severely downgraded lacking lots of the droids and placeables from the earlier version making it feel empty and incomplete, while I can understand they are the cause of the lag I felt that downgrading the level severely was somewhat uncalled for, cutting down was inevitable though I felt they went a bit too far. Not that I'm complaining about my less laggy experience though.
The new ending is kinda weird, and by weird, I feel it is bugged. You can go to the environmental zone first, fight the droids and convince Kaah to help you get to industrial archon to reach M4-78. I'm loving the freedom of choice with that though of course, this method is somewhat hidden (You need to use Bao Dur's shield breaker to break into the environmental zone). What I don't like is how Darth Sion on Korriban is tracking the Exile as he does on Malachor in cut content with the meditation and the hologram (Which is broken for females). On Malachor he says 'prepare my ship, I'll attend to this myself' but here he instead sends the Sith Assassins.
The Sith Assassins, with basic attack and no force powers, kill Kaah and Vash in an ambush and once this ambushed has happened I fell that is where M4-78 ends. I much preferred the original ending as it felt more... like an actual ending and it had an actual resolution with M4-78 and the droids as well as Kaah and Vash in a believable way. Not that the new ending can't be bug fixed and enhanced, however.
For 1.5 that's all I can think of. For M4-78, in general, I dislike the infinite medical equipment droid merchant and the inconsistencies with the environmental zone droids.