The game is OK at best, could be a lot better though.
-Simple graphics, tutorial, and interface help beginner tower defense players learn quickly.
-Fun, simple gameplay, with some strategic depth.
-Great difficulty curve that forces change of strategy on later levels.
-Many units are generally well-balanced and useful in their own way.
-Decent value game if you spend $6 or less.
-Low replay value; game is beatable in 3 or 4 hours, nothing else really to do other than survival mode and minimal achievements.
-Suicide bomber enemies are OP. I would understand if all units die from bombers in one hit except for tanks, but even low-level tank units get killed by them in one hit. Only max-upgraded tanks can survive one blast, but with almost no HP left. Enemy suicide bomber damage should be reduced a bit so that level 1, 2, and 3 tank units can take more than one bomber hit, but no more than two, before death.
-Basic archer units should do a bit more damage.
-No way to see specific enemy stats, or how your unit buffs are affecting unit stats.
-When building units, no name indicators are given when choosing what unit to place. There needs to be names shown when mousing over the units, so that you can distinguish between each unit without incorrectly selecting the wrong unit to build.
-General bugs ruin gameplay: enemy units go right through defences when they shouldn't in certain levels, sometimes certain areas that you should be able to place units on become unbuildable during gameplay, bad melee AI makes them move to areas that you don't want them to move and attack units that you don't want them to.
-Limited music becomes tiresome to listen to.
-Unit placement voices are annoying.
-Barebones storyline and graphics.
Overall, 6/10, maybe 7/10. There are many better tower defense games out there (Dungeon Defence, Plants vs. Zombies, etc), but I would recommend this for family gamers, if you're looking for a budget TD game, if you're a huge TD game enthusist or if you're a TD beginner. Otherwise, look elsewhere.