Well I'm partway through it
It's a bit boring and tedious. It takes hours even on 3x speed to get the good stuff. You should make the levels shorter. However I do like that you get to vary how hard the levels are by deciding when to get the next wave. That makes it so you can basically always get through the level if you just wait, you only die if you feed yourself more enemies than you can handle, by your own lack of judgment.
Here's the deal on the towers. You can roughly calculate the efficiency of a given tower at a given upgrade level, as Rate of Fire times Damage divided by Cost. This assumes the tower is able to fire constantly.
Of the "conventional" towers--fast tower, basic tower, and super tower--the super tower is by far the most efficient. Once you are past the first few levels, it makes sense to just make super towers. As for upgrading them: the first upgrade for the super tower is less efficient (relative to its cost) than no upgrades, but the second upgrade is equally efficient as no upgrades, and after that more upgrades for the super tower make it more efficient. So you should usually upgrade your super towers instead of making more of them, assuming you can do at least two upgrades.
The multi fire tower is useless also. Even if you assume that all 8 of its bullets hit enemies on every shot, it's still less efficient than the super tower.
The poison tower is interesting. The total damage it does to enemies isn't given and depends on the length of the track. For a poison tower with no upgrades, and a medium length track, I estimate it can do about 300 damage over the entire duration. That would give it an efficiency more than twice the super tower, at my current level of "Towers Damage" upgrade (level 10). However, in order for it to do that much damage, your other towers can't kill the enemies until they are almost at the exit, which leads to an awkward strategy of several poison towers at the entrance and a super tower or two at the exit. Also, upgrading the poison tower makes it less efficient relative to its cost. So i think the poison tower has potential, but probably it's not worth it for most levels.
The slow tower is junk, and I'll tell you why. To get the "expert" rating it doesn't matter how slow the enemies are moving. It's just a question of the rate at which enemies are entering versus the rate they are being killed. You don't have a "pause" between waves for you to mop up slowed enemies (doing that takes too long so you won't get the expert rating). Slow towers help in other tower defense games, but not this one.
To get the expert rating on levels, if you haven't figured it out: play at 3x speed, and put a few super towers right near the entrance. Your goal is to get every super tower you have firing constantly, but no enemies spilling past. If any of your towers aren't firing constantly, send another wave or two. If any enemies are spilling past, don't send more waves, and hope that you can mop them up before they get to the finish. It's best to keep the number of super towers small, but upgrade the ones you have.
There's one thing I might want to try, and that's using bombs. In theory, you might be able to send all the waves at once, and then use bombs to destroy them all at once, because each bomb would hit like 30 enemies. But you'd need the upgrade to make bombs to 10x damage, and probably the other bomb upgrades too, before this would work. It would be pretty tedious to get all the upgrade points for that.