I dig it. Any program that measures caloric output in terms of slices of pizza is cool in my book.
The major problem with the trainer is it's boring as shit. You just sit there and pedal and try not to think about how awful it is. The second major problem is there's no real way to track progress, over and above time, which simple time ridden is a metric that I'm not that interested in.
Zwift addresses both of those. Even just having a virtual world to ride through, and seeing scenery change and having discrete routes and loops you can ride, really helps with the mental engagement side of things. Then there are the in-game features like timed KOM hill climbs and timed sprints, and a couple loops with timed leaderboards as well, and having a timer start ticking is a pretty awesome motivator to get out there and hammer. It also integrates neatly into Strava, so you get Strava segments and your virtual mileage is included in your total annual mileage, etc, which is also pretty cool - the segments, especially, give you a great way to see how you're trending over time, so if you really hammer a sprint segment one day, not only can you see on Zwift that you set a new 30-day PR, when you get off the bike on Strava, you can see that you've also set a new overall personal PR, and how you stack up against riders all over the world (which, on the Watopia island main loop, oftentimes is now more than 260,000 riders). That's cool.
As far as smart vs conventional trainer... I rode on Zwift for maybe a year with a Kurt Road Machine before splurging for a Wahoo Kickr. The Kickr definitely improves the realism and IMO is a beneficial training tool - the winter I spent on my Road Machine, I felt like my steady state power was pretty good but I wasn't really able to similar getting out of the saddle on a steep climb with that kind of resistance in the cranks, so when I hit a steep climg for the first couple weeks of the season, it felt like I hit a wall. The first year I was on the Kickr, I felt pretty strong. That said, both years I was riding way better than I was before I got on Zwift when I spent barely any time at all on the trainer. They calibrated their Z-Power estimated power monitoring using rhe Kurt, and IMO I didn't see much in the way of substantial differences in my measured power output between the Kurt and the Kickr, so you can definitely use it with a passive trainer and not be at a disadvantage (or unfair advantage), just make sure it's one of the supported ones. But, IMO, the smart trainer does add a lot.
You definitely are going to want a fan, though.
I guess the tl;dr version is Zwift is definitely
worth the $15/mo they charge as a way to add a lot of interest to the trainer. A smart trainer adds a lot to the realism, but is also a pretty big expense, especially if you already have a trainer Zwift supports. For me, I'd do it again, but that's a question that will vary a little more from person to person.