I picked up the Ultrabundle for $3 or so, definitely a good deal even though I already owned Revenge of the Titans. However, I wouldn't recommend this game (or the bundle) at normal price.
Titan Attacks is quite enjoyable. I played through the first 2 worlds a few times, learning the game and practicing, usually getting destroyed on a few key levels. However, by a few hours in, I broke free. See, the difficulty curve is ramped in the early waves, while you are still building upgrades and gaining power. The game seems to break a bit once you hit Mars, when you usually get maxed add-ons and bullets. After that, what was once a strategic shooter with 1 bullet on screen becomes a spammy hellzone. Waves last a few seconds, cash becomes trivial, even bosses and damage cease to inspire tension. By my 5th new game or so, I could beat the game almost twice. There is no ending, just immediate New Game+ and scaling enemies.
It didn't make me feel like a pro, it made me feel like the game was incomplete. 100 simple levels, menu music that loops after 5 seconds, and so much potential.
It's not all bad. The gameplay and shooting is pretty solid, sounds and music are good, and the visuals are solid if a bit murky in some dark red areas. For $3 or so, a solid experience.
Compared to Revenge (even though I HATED that game) this game feels very simple and short, beatable in an hour or less. Once I finish all the achievements, I probably won't mess with the game much. I'd prefer if the game had more strategy developed in the later levels, nothing really changes. The only real source of difficulty is avoiding a screen full of tiny bullets while enemies just gain HP as the waves go on, slowing the game down without adding any challenge beyond "Fire more". It started great, became decent, and ended with a bigger version of the same boss. I guess it's fair, sometimes it's nice to be able to beat a game and move onto something else. Just watch for a great sale. Now to work on Ultratron and Droid Assault...