Charming dialogue and wonky control, lazy visuals and a lackluster story. A decent game but not Mario's finest RPG.
Adding baby versions of Mario, Luigi and co. to the plot was a bad idea in my opinion. The cast has a disproportionate amount of annoying characters and there's far too many cutscenes between the four main ones that set out to be funny or cute but go on too long and try your patience. You aren't going to find a heavy story here, the time traveling plot is pretty linear and not as enchanting as I was hoping for. There is plenty of well written dialogue and it's a great localization, although I stopped caring long before the game finished. Too often scenes feel needlessly drawn out, and I found myself wanting to skip through a lot more of the game than I was allowed to.
On the surface, the gameplay has many similarities with previous Mario RPG's. There's timed attacks during the battles, emblems to collect/wear and puzzles to solve with an expanding arsenal of actions. Unfortunately it's a less satisfying experience than before due to the fact that controlling four characters at once grows tiresome and makes puzzling and navigating feel like a chore. The more skills your team learns along the way the more convoluted it gets to control, which may be a plus to some but came off as unnecessary to me. Battles grow repetitive but the timed button pressing still keeps things interesting and it's probably the most fun part of the game. Boss encounters are especially inventive. You can customize your characters with a few emblems and after gaining levels, but it's not too deep of a system.
Graphics are a mixed bag. The game has a wonderful Nintendo polish to it, with lots of familiar Mario touches. However despite a few nice effects, most everything on display could have been faithfully created on the GBA with power to spare. The game is plain looking and borderline ugly for a DS title. Character sprites exhibit personality but lack detail, the environments are varied but most aren't that interesting to explore. The pacing never feels quite right as you move around the various worlds either, and you must warp to reach the different levels - there is no overworld in the game.
It's worth noting that Partners in Time doesn't use the touch screen, even when it would have greatly improved navigating battle menus, inventories, and exploration. The dual screens are used frequently in cut scenes and partner switching, although the constant shifting back and forth got on my nerves much more than it immersed me in the gameplay. The game misuses the features of the DS in my opinion, hopefully future RPG's for the system fare better.
Mario & Luigi: Partners in Time is not an overly memorable experience but it is a long one and can be addicting, as is the case with most competent RPG's. It's a game that does some things well and others poorly, and the fact that Partners in Time is simply a capable title and not an excellent one is amplified in the shadow of it's more entertaining older siblings. If you're looking for a Mario RPG on the go, stick with the GBA original. This isn't the AAA DS roleplaying experience we've been waiting for.