It is a substantial surprise to note that I am only the eighth person to register having played Custard & Mustard's Big Adventure. The fact that this game lounges in obscurity is a crime.
Let me assert, as a counterpoint to Mike Russo's review, that I came to this game predisposed to not like it. As a fan of IF with command prompts, I admit to a certain prejudice toward any parser game not written in one of the "major" systems (i.e. Inform, TADS, Dialog). The shortcomings in the platform translate to shortcomings in the gameplay experience, and there's a bit of grating annoyance every time I run into the kind of problem that is just automatically "solved" by using more developed systems.
That said, an author of sufficient skill who is willing to take on the burden of overcoming those shortcomings can manage it with enough work, and it's clear that Christopher Merriner is one of those who has done so.
I say this in all seriousness: Custard & Mustard is the funnest work of IF that I have played in years. The last time I enjoyed myself this much was playing J. J. Guest's Alias 'The Magpie', and the style of humor here is a cousin (if not quite a close cousin) to that marvelous farce. The main difference is that where Magpie leans highbrow, Custard & Mustard leans lowbrow -- though it does this without embracing crudity or mockery. This is terrific family-friendly fare, though as noted by others it's likely that younger kids will need some assistance to solve certain puzzles.
To the extent that this game has flaws, some of them are deficits of Adventuron which simply must be conceded (such as less definition between various types of parsing parsing errors), and some are design choices that could be revisited (like a reduced level of handholding in orienting the player to the plot or the need for repetitive actions to complete some puzzles in a couple of places). The >HINT system works well when needed, more so in the later game than the earlier, where it seemed to often have no suggestions and the "solution" tended to be to just wander around the map until a cutscene is triggered. (This is less of a chore than might be expected, as the rich object implementation offers a cornucopia of amusement in the form of observational and situational humor. As Russo notes, the protagonists' sense of smell is an ever-present companion, and the verb >SNIFF should not be forgotten!)
However, on the whole the design of Custard & Mustard is exceptional, tightly interweaving various lightweight puzzle-oriented set pieces with the consistently entertaining story. The imaginary small town comes alive through a series of vignettes taking place over the course of the day, and a sizable cast of secondary characters will impress themselves on the player's memory though huge amounts of "business" in the form of background actions and conversation. Flipping back through the various scenes in my memory, I'm astounded at the amount of work put into this aspect of the game -- it must have been difficult, but it was very worth it, as it nearly always seems like the model universe has more to offer than one is seeing.
I'm going to hold off on a rating for this unforgettable romp for now, but it handily reaches four star territory and is a definite contender for a rare five. Tip of the hat to Merriner for this outstanding crowd pleaser, which is the newest addition to my "games to recommend for kids and newcomers" list.