My footing on this topic is less sure than most. The only game in the series I have put any serious effort into playing is Persona, and I'm backing that up with some standard poking about at screenshots, reviews, and discussions with people. It's entirely possible that Persona gave me an unfair view of the series as a whole. So really, what I'm reviewing here is Persona, with comments on the series as a whole honestly restricted to inferred generalizations. That said, Persona is generally considered a good representation of the series as a whole, so these thoughts should be accurate enough when applied to the rest.
The premise of the Megten games is to have some RPGs set in the real world, with plots dealing with metaphysical stuff, and choosing between good and evil. Of course, angels and demons fighting isn't exactly a rare subject for RPGs, and that the games are so surreal that the setting doesn't matter much, so I find it loses impact as a gimmick.
Here's what really jumps out at me when I see these games. Sterile minimalism. In a typical MegTen game, our map is a simplified street map, and our party is represented by a little green arrow. Cut scenes may be an isometric view of your party members standing around like during a fight in Breath of Fire on a sparse background over-layed with some talking heads. Combat looks similar, but with less of a background. Then of course, if Persona is any indication, we have a nearly non-existent soundtrack added to the mix. Quite early on in the game, I spent quite a while in a maze where the only audio was a short loop of odd laughter, and footsteps.
So they aren't pretty as a whole. Not usually a major concern of mine, and more recent entries seem to look a bit more interesting. My first concern with any game is that the actual gameplay is engaging. Here though, I get the distinct impression that the meat and potatoes were hastily slapped together overall. At least, the important bits.
Heck, I'm just going to come right out and say it. The MegTen games have first-person mazes, and first person mazes suck. I'm not saying they're always a bad thing. There are a handful of RPGs out there with first person mazes that I find quite enjoyable. The good ones though use them for a reason. It's either an intimidation issue, or they want you to make sure you're thinking while you're exploring, or it's a nice change of pace from the rest of the game's graphics. The rest of the time, it comes off as a cheap trick to make your graphics seem really impressive without having to do a whole lot of actual work. That or you're trying to hide the fact that all your mazes would fit on one screen with a typical top-down perspective. In any case, the bad examples (like we're discussing here) have two major earmarks. First, there's nothing all that threatening about these mazes. Second, off on the side you have an ultra-simple top-view map.
Looking at the maps of mazes in Persona brings me to my second biggest gripe. They look randomly generated. All over the place, there are one or two space deep dead ends, which never have anything in them. There is no reason anyone should ever walk down these, either in terms of reward, or in terms of being tricked, so why have them? To make it look more like a maze. The advantage to a game which actually has randomly generated mazes is the constant surprise, and inherently fluctuating difficulty. The trade-off here is that anything else remotely interesting (interesting layouts, hand-crafted detail, decent variety) are lost. Here, we have the worst of both worlds.
Once again though, I said this was my second biggest gripe. The biggest, is the main gameplay gimmick. If you read a list of bullet points about any given MegTen game, they'll make this big deal of how you always have the option of talking to monsters instead of killing them. Technically, this is true, but the implementation blows. Every type of monster has a different list of topics to pick from when you talk to it, which amount to picking a random option off a list. Then either you wind up fighting anyway, they give you a worthless item, or they wander off. From where I sit, it's just rather time consuming, and to truly take advantage of it, you need to compile through trial and error (or go look up) a database of what happens when saying what to every monster there is. This may have appeal to walkthrough writers, but to me, it's tedium.
This is typically mixed in with one or another form of Pokémon-esque gameplay. Monsters can be recruited, or give your their souls to equip, or something like that. Either way (and again, this is something that may apply less to the rest than Persona) we have one of these systems where I'm constantly picking up new toys/pets, having to upgrade them for a while, then eventually being forced to toss them out (or mix them together) to make new ones, and repeat. Once again, this isn't my usual idea of fun.
So, we have slapdash game design that doesn't seem to have much serious thought behind it, backed up with spartan graphics and music. All that leaves to be interesting is the plot, which, while interestingly surreal, doesn't strike me as being worth the effort.
Why then does is this series so highly rated? Well, there's a certain sort of person who enjoys these things I've been calling tedious for their own sake. There's some people who think anything controversial (in this case, defining controversy as making statements that would likely offend devout Christians) must be good. Then the surreal plots make good bait for the sort of person who praises the stories in videogames. You know, the sort who pounce on anything remotely thought-provoking and proclaim it's the greatest thing ever written, despite usually having no real basis for comparison?
Less pessimistically though, there's a good deal of variety to the series. Each little sub-series seems aimed at a different niche market, so odds are there's going to be one game you'd find enjoyable in the bunch. Personally though, I just tend to steer clear.
Curious about something else? Ask me what I think.