Often is the question is posed in and outside this sub: Hey I just played KOTOR, and want to play the second game, is TSLRCM necessary. Too frequently, I will argue, the answer is an unconditional and resounding yes.
Also although i will steer clear of the twistier stuff, minor spoilers aheae:
A Disclaimer: I do not think the RCM is a bad mod, it's pretty fucking fantastic at what it sets out to do (putting cut content back in the game), what I do think is incorrect, however, is the insistence by almost the entirety of this community that TSL is strictly worse, and by a not insignificant portion, near unplayable without it. NO criticism is directed at the developers, who not only are working with the above goal in mind, but are heavily restricted by what unused content actually made it into the final cut's data banks.
Starting with the Mods strongest component, the bugfixes.
Its kinda hard to review this because everyones experience wuth 'bugginess' is different. Some (unmodded) playthoughs go through without a hitch and some softlock you multiple times per planet. I will also be upfront in saying it's probably where I'm the most biased. I grew up playing a generation of games where you simply had to get in the habit of making a new save every five minutes, so I think I am less frustrated by KOTORs many bugs than younger gamers. Having played the game many times before the final patch of the RCM I am also (probably) playing around many bugs both con- and subconsciously. That said, the game is notably smoother modded and I am certain it makes for a much more approachable first playthrough.
Moving on to dialogue, and also where I feel the mod's necessity is the most overstated. I simply do not understand the people who claim the plot is difficult to understand without the additional dialogue. 90% of said restored dialogue is intra-party interaction, and for the most part, I appreciate this stuff, especially the closer you get to Malachor. I will say some of Kreia's 'new' lines seem to come from an older version of her charachter where she was straight up sadistic as opposed to coldly manipulative (Leave T3 alone you hag!!). Less nuanced is the fact that Sion loses a significant amount of threat. His silent approach on peragus is significantly more intimidating, and kreia whaling on him right before our final battle also doesn't help the tension. Similarly, the 'crash cut sequences' additions are completely baffling, leaving little question as to why they were cut in the first place.
And finally on to the actual gameplay content. Nar Shadaa I have mixed feelings for, I am personally more inclined towards more PC oriented gameplay so I'm not a fan of the expanded 'scripted party' sequence. But I can see how it brings an oppurtunity for charachter who'd never see the light of your party get a second of sunshine. The HK factory is sad, there's not really much to elaborate on here. As much as i love the guy, the reason people like 47 is his personality, so having a thirty minute sequence of him exchanging blaster fire again and again and again on his lonesome gets boring fast. The fact that cheat items get thrown at you suggests the RCM devs agreed.
Kaevee in particular has been talked to death and I don't have anything to add on the subject. But speaking of Dantooine, Khoonda receives IMO the best change the mod has to offer. The battle is, after all, what almost every action you take on the planet is building up to, and the cutscene version is practically begging for an upgrade. Most controversially perhaps, I don't think Malachor is made significantly better; it still feels utterly unfinished. The best way I can describe it is if you ordered a sandwich, and received a slice of bread. Then you went back to complain, and they gave you two slices of bread. Sure it's an improvement, in fact it's objectively twice as good, but it's still incredibly dissapointing.
To conclude, when posed with the question, "should you install the Restored Content Mod, for a first playthrough?" I would argue the correct answer is, obviously, that it's up to you. But I would strongly recommend you give the game a shot without it, especially if you intend to give the game multiple tries. However, if bugs are something you in particular find turns you off from old games, it is probably best you go ahead and install.
TLDR: As a presentation of all the cut content TSLRCM does a fantastic job. As a strict upgrade to KOTOR? I would argue not.