My feelings on this DLC are kind of complicated.
I have really wanted to have the chance to build or renovate a vault since the very first time I played Fallout 3. When I was a bit younger, I actually once did a thorough cleaning of Vault 106. I meticulously picked up every single piece of trash (every aluminium can, every burnt pre-war book, every empty Nuka-Cola bottle) and hauled it to the surface so that I could role-play that I was bringing Vault 106 back online and turning it into my base of operations. The restored Vault 106 would surely shine as a beacon of hope in a weary world! Alas! 'Twas only a fantasy, for even though it was emptied of junk, it still looked (and probably smelled) like ass and completely lacked any safe storage or any beds that I could use to become well-rested. I did proudly wear the Vault 106 jumpsuit as I ran around the Capital Wasteland, however. So it should come as no surprise that when I learned of this DLC, I was so excited that I probably became totally incoherent. Finally my chance had come.
Overall, this DLC is rather disappointing. The vault-building is just not very engaging. You get to build the vault inside of a large cave system, which sounds fine until you realize that the footprint that you have to work with doesn't exactly lend itself to this type of construction. You also don't have the space to really get creative with the layout or the design. And because you're building the vault inside of a cave, the ambient lighting and dustiness inside of the cave persists inside of the vault sections after you place them. It's not a pleasing phenomenon. It just looks odd. If you persevere and build out the vault, then you will inevitably find that the settlers whom you invite into your beacon of hope will tend to congregate in random locations (stairwells, hallways, the overseer's office, public restrooms) for no obvious reason, even though they all have jobs to be doing. Even if you build a vault, the shape of the cave guarantees that it can never really be finished. On top of all that, the vault segments you have available to you are not anywhere near as elaborate or interesting as those found in the other vaults in the Commonwealth and Far Harbour (75, 81, 95, 111, 114, and 118).
If you are like me, and the idea of building a vault is really interesting to you, I want to make sure you know what you're getting yourself into. Expect to spend long hours tinkering with the vault sections and with Nexus Mods (an absolute must if you're determined to build a halfway decent-looking vault). Also be aware that Vault 88 is never going to look anywhere near as good as Vault 81, or any of the ruined vaults in Fallout 4. I go back and forth with this mod because I keep forgetting how bad Vault 88 always looks and thinking "I just need to give it another shot and try harder." No amount of trying harder will fix the problems that Vault 88 suffers from. I get the biggest kick out of Cait's comments whenever I take her into Vault 88. She says the same line she uses in the other vaults, like "This place is a complete joke. Vault-Tec were a bunch of stupid morons," or words to that effect. I always wish I could go, "Really Cait? I'm standing right here. I built this place with my own two hands."
The story arc in this DLC is one of Fallout's more bizarre episodes. The characters act in ways that aren't really believable throughout most of the story. Vault 88's ostensible overseer, Valerie Barstow, acts like a comic book villain, and the settlers she lures into the vault to participate in her experiments have got to be the three biggest tools in the entire Commonwealth. Many of the objectives seem largely pointless, although the experiments (particularly Barstow's and Clem's reactions to the positive karma outcomes) are funny.
I really think this idea can work, but it should have been done differently. I imagine it being a bit like the Promised Land side quest from the beginning of Star Wars Knights of the Old Republic. Maybe you meet an engineer in your travels who has evidence that one of the vaults in the Commonwealth could be potentially reactivated if only its current residents could be evicted (raiders, deathclaws, synths, Mirelurks, Gunners, whatever). So you join them to clear out the vault and start fixing it up and bringing the utilities back online. Maybe you have to recruit a vault doctor and some prospective vault residents.
So in summary, Vault-Tec Workshop may be worth the money (particularly if it happens to be on sale), but is unlikely to be worth the time or effort (unless disappointment floats your boat). If you see it on sale, you might as well pick it up. You do get quite a bit of new stuff for building this settlement and other settlements, which is nice. A lot of the other people I have seen have just ignored the vault-building option and built normal settlements inside of the cave system. I see some other reviewers have mentioned the population management computer for assigning jobs to settlers, which is very useful.