The basic gameplay is easy to learn and rather free-form. Your main 'goal' is to explore new areas by telling the Diggles to dig tunnels. Along the way, you will meet new dangers, challenges, or characters that help advance the plot. Like other RTS games, resource gathering and management is key to winning the game, since the Diggles need to eat, multiply, and feel content enough to work.
The slow pace is the main problem that will turn people off. I consider myself a patient person, and even I want to poke these Diggles with cattle prods so they'd do their jobs more quickly. Sometimes they react to commands so slowly, you will wonder if there is a problem with your mouse. When the Diggles start working, it is often difficult to guess when the task will be completed. Part of this is due to the Sims-like quality of the Diggles: similar to the Narns in Creatures, they have different "personalities" which affects how diligent they are, or if they even want to work at all. Each Diggle has a complete set of statistics and inventory, which you have to monitor constantly. You can set the amount of time each Diggle spends working and relaxing - their "leisure time." During leisure time, the Diggles sleep, eat food (like roasted hamsters), and play - watching them do handstands, knit, and compete in karate tournaments is quite fun, actually.
The game does allow you to crank up the speed to double and quadruple time, but at these speeds you have no control over what's happening, and therefore these options aren't very useful (except when you start huge digging projects, which is rare). The slow game speed does make it easier to keep track of everything later on once you have dozens of Diggles to order, but this extreme micro-management is sure to annoy most gamers.
On the upside, the cutesy and crisp graphics is one of the game's best aspects. It's like viewing an ant farm in 3D, except with exotic creatures and much more detail. You can pan, rotate, and zoom in on the action easily, and the animations are detailed enough for you to know what each Diggle is doing. Also, the campaign mode is quite interesting and full of memorable characters, although the lack of multiplayer severely limits replay value of this game.
When all is said and done, this cross between Red Alert, The Sims and Creatures come off as a mixed bag. The clever blend of ideas from multiple genres and the unique setting are good points, but the game is too bogged down with tedious micro-management to be addictive. There is far too much micro-management, all of which take a long time to execute, and the game's lazy AI and finicky mouse controls make this problem even worse. The game does not lack depth (pardon the pun), but the execution leaves much to be desired. In summary, Diggles succeeds admirably at engaging gamers' attention, then fails to entertain them. Recommended only for die-hard fans of micro-management or cutesy Tamagotchi-style life simulations who have more patience than yours truly.