I used to play a game in high school called Motherload. You had a little ship with a drill and you could drill down the earth to find treasures, but it was dangerous. You had to be careful about bumping into the walls and floor, you were overheating constantly and running out of oil was a death sentence.
I Dig It is almost the same game, you need to drill down, find stuff, go back to the surface and sell it to buy more stuff. It's a very easy to understand loop, you always do the same things, but as time goes on, you become better at doing them and your rewards grow, allowing you to be even better and getting more and more rewards.
When I started the 'campaign' I was a bit puzzled by your ability to choose between multiple levels, I would've understood if you got from stage 1 to stage 2 and so forth, but already I feared that the concepts of the game were going to be used in weird ways since you could choose any map. The first mission was simple, collect 50 000$ in two hours. First of all, the time limit and the money limit made me raise an eyebrow, upgrading your drill ship is the fun part of this kind of game, not racing against the clock!
After I got the 50 000$, my fears were confirmed as I started the next level with no money and no upgrades at all. The idea of having to restart every time didn't encourage me much to try progressing through the campaign and that's understandable, people don't like losing their progress, when they get some cool upgrade, removing it from them isn't interesting, why not make the campaign more linear, tier the upgrades (you can get drill level 3 max by level one, level 5 max by level 2, etc.) and keep them as you go?
Getting back all of my upgrades was tedious, but I pressed on a few more levels before deciding that the campaign mode wasn't for me. The controls were okay but having the slowest drill and no cargo space (and no fuel capacity, etc. etc.) every time didn't make for a fun campaign.
The game tries to add more depth by recording everything you collect, and it's a nice touch, but a meaningless one. It's fun to see that you collected a thousand rocks, but adding a purpose to that would make collecting these rocks more meaningful, maybe making them worth more? Easier to dig? Some achievements are related to collecting certain rare items, but they are very specific to some type of collectibles and you won't get them by accident.
There are also extra modes, but I was confused when I saw that the 'free dig' mode gave you everything upfront. This is the mode with no timer, no cash limit and no goals, you can play it in any stages with the challenges associated with them (such as giant ants) but there's no point, you already start with everything upgraded to the max and you can dig for no reason. Why not have the player upgrade normally?
All being said, there are a few good ideas in I Dig It. The levels underwater where you don't have oil to worry about but air and water pressure are fun, some of the challenges (like collecting coins with a time limit) are an interesting twist on the basic gameplay concepts but the main campaign isn't fun enough to play with.
While you couldn't keep upgrades coming with the basic 5 levels of hulls/drills/storage, there's space to add more mechanics to that kind of game. Why not have some small base-building elements where you can create basic buildings to refuel in/unload your cargo and then link all of that with the surface? That'd be interesting. Or skill trees where you specialize your drill machine in things such as drilling rocks or tires. As-is, I Dig It is a modern, less addictive Motherload. I don't think I'll play much more of it.