I've had it since 2004. It is the greatest thing ever if you're out of market. EVER.
It can be a bit hiccupy at the start of the year, especially if they've changed things around on the back end, but they're always sorted out very quickly (IMO, a lot of the problems are often from the user's ISP or device being unable to handle the load rather than MLB.TV itself). They're hardly the only sport that has this problem, I remember when UK NFL fans practically revolted due to NFL GamePass being completely unusable for the first two weeks of the season a few years ago.
The apps on iPad and iPhone are amazing, being able to prop up my tablet while doing dishes and watch a game is fantastic. I actually watched the last few World Series on my Roku box even though I have an HD channel with MLB because the quality was even better. Lag on those devices is very uncommon.
I'd highly recommend getting the whole package, as noted you'll end up watching so much more baseball because you'll end up checking in on day games, no-hitters, this guy has 3 homers and it's the 7th inning type situations, whatever's on. It's just unbelievable for that - I think Keith Law once said he just scrolls down Twitter and monitors developing situations and then checks into that game. It's like RedZone except you're the director.
The only problem is blackouts. I don't get any in the UK and I can empathize with the cable situation because cable pays a lot of bills*. But goddamn, it was really annoying being in Vegas last year and being told I can't watch the Giants even though they are 100% not available on local cable, much less hotel room cable (I checked). I believe MLB.TV lets you plug in your ZIP to see what teams are blacked out - definitely worth checking if you might live in an area where there's multiple teams blacked out. In that sense, unless you're willing to go the VPN route or you 100% do not care about not being able to watch local games, it's really a complement to cable rather than an alternative.
* Back of the napkin calculation on why cable blackouts are never going away: NESN charges something like $6 a subscriber. That's $72 a year, 80% goes to the Sox (the Bruins have 20%). Plus they get ad money, which I think is another few bucks a month per subscriber. So let's say it's $8 a month in total. That's $96 a year to NESN thanks to you. MLB.TV is $120 a year and that has to be shared across 30 teams. Yeah.