The comparisons to Mine are useful, but they don't tell the whole story, as the thing that makes Taxman interesting is the attack. In some ways it's a lot like Cutpurse, without the immediate cash value, but it also attacks Silver or Gold.
So Mine has some problems. It's decent early game, and the benefit it gives you lasts for the duration of the game (more money density). It's slow to resolve, and as the game goes on it gets progressively less useful as your money is mostly upgraded. It's a terminal Copper the turn you play it which isn't that great.
Taxman doesn't fix all of those problems, but it addresses some of them a little bit and it's specifically a lot more useful later in the game than Mine. So as a $4, you get to start using Taxman a lot sooner, which actually helps a lot. It's an attack, so while you're taking weaker turns by using Taxman over, say, Smithy, your opponents are also a bit stunted to compensate. In many situations, enabling a bigger next turn is better than a bit of help this turn with no attack, but in return your deck cycling is slower.
The real strength of Taxman is that it can still be good in the middle to end game, particularly against an opponent playing with Treasures as the main source of income. If you're careful about deck tracking, both your own and your opponents, you can trash Silver or even Gold to neuter your opponent's turn, if you think it will do more damage to their turn than it's doing to your turn. If you have a $7 hand near the end of the game, you're already buying a Duchy, so you might as well Taxman the Silver for the attack. This isn't a GREAT attack, but it's got a bit of niche utility. I think the biggest factor in determining if Taxman will be useful is evaluating how strong the attack will be near the end of the game.