I'll try to keep this review short as there's been some great in-depth criticism from other people already, but I really felt that I should at least comment on how good this game is. Actually, I registered on RMN just so that I could review it. It's that good.
Honestly, I haven't had that much fun playing a RPG Maker game since ... well, I can't remember clearly, but I believe it was around the time when mullets were fashionable. Or was it togas ? Maybe both.

Gameplay : 4/5

I came for the story, having read the other reviews, but I did end up liking the gameplay as well. Like calunio I'm somewhat tired of standard RPG fare, but I believe this game is short enough for it to work, as the nice little variations on the default RM2K3 system provide some renewed interest even for someone as blasé as I appear to be.
Well, you know everything there's to know about the gameplay halfway through the game, so it does become a little bit repetitive in the last parts, but really, the length and difficulty are just right and the boss battles are quite fun.

I skipped most of the random battles and was still able to beat the game (with just enough challenge to make it interesting), so it's rather nicely balanced if you don't grind at all and just fight the battles that you couldn't avoid.
I was also a bit annoyed by the backtracking in the Marsh, especially because it was difficult to avoid the fights there, and I put up with it mostly because I knew the end was near (thanks to the soundtrack listing which I used as a progression meter), but frankly, nothing ever killed my fun.

Presentation : 5/5

This game is very, very nicely crafted. I agree with others on the excellent musical choices, which enhance both the general atmosphere and the feeling of urgency in the gameplay during boss battles - I am among those who loved the Queen B theme, it was perfect for the occasion !

The graphical aspect is very consistent despite the diversity of sceneries, and always nice to the eye - even dungeons mostly manage not to be too repetitive.
The best thing is that graphics, music and writing are very well welded together, and I loved the way that cutscenes, chapter beginnings and ends and other narrative moments were handled in this respect.

I won't expand much more as many things have been described in other reviews already, but I really appreciate the work that has been put into this aspect of the game.

Writing : 5/5 (Very vaguely mood-spoilerish)

Undoubtedly the strongest point of Leo and Leah, and the one which really kept me going through the game. There isn't much to say about the story, as it is a voluntary cliché and you'll go through a castle, a swamp, a pirate ship and almost every single standard RPG location you can think of (it is lacking in ice caves and volcanos though, thankfully). However, it's the good kind of voluntary clichés : not "you'll see, everything suddenly becomes amazing 20 hours into it when you learn that the wizard had it planned all along" , but one of the multiple ways this game plays with the player's expectations, and a pretext for interesting interactions and ambiances.

And indeed the atmosphere is really well crafted and never disappoints, from one end of the mood spectrum to the other (well, maybe a few elements near the end were a little underdevelopped, like the monkeys and bowers, but otherwise everything seems to have been given much attention)
I did see a strong parodic element in this game, though mostly in a tongue-in-cheek way, and it came off great as even the most straightforward caricatures had little spins on them which prevented them from ever going stale.
There is fourth wall breaking aplenty, and some nice ones at that, even though I didn't think I could still be excited by that kind of things as it's more a tradition than a revolution in videogames. The end of the game makes an especially pleasurable use of it.

I loved the secondary and tertiary characters, and all the brilliant little ideas that pop up everywhere in this crazy, crazy world (the signs and cats are of course a personal favourite, and I could never resolve to eat the poor yummyhops). The NPCs were always quirky and I was compelled to talk to everyone and explore everything, something which I hadn't done in a RPG Maker game for a long long time.
In contrast, I believe the protagonist *had* to be silent : it allows sudden distancing from his actions, as we realize from time to time that we have no idea what he's thinking right now, and aren't even sure that we want to know. This contrast with the more explicitly characterized NPCs is used to great effect at some points in the game.

Some mood changes were a teeny tiny bit more heavy-handed than others, but overall it was done in a consistently clever way, and there were awesome WUT moments as events briefly took a turn that I never expected, then continued as if everything were normal. I can see why this game could be qualified as "subversive" among RPGs : heroism and the will to do anything for the one you love are depicted in a rather ... ambivalent way. And although this is isn't the first time I see such a take on the usual heroic tale and RPG conventions (and quite persistent cultural clichés about love and whatever), it hasn't lost much of its impact, maybe due to the fact that it appears in a very pervasive and gradual way throughout the story.


Overall

This game is chock-full of great ideas, clever little touches, and just plain old fun - also, it's just the right length and format for its purpose. It elicited more feelings and interest in me than most RPGs I've played these times, both amateur and commercial. I never played the Mother series, despite every good thing I've heard about them, but this really made me want to try them some time.

Final word : To the author, thank you for this great game ! It's among the finest RM productions that I've played, and one of the few I would consider showing to someone outside the community as an example of what can be done with the medium. So thumbs up, or paws up, or whatever !