Pro

• Solid gameplay.
• Amusing context/story.
• RPG-elements in are underused in Tower Defense games.
• Nice graphics.
• Squad members all feel distinct.
• Decent variety in towers and enemies.

Con

• Clumsy controls, you'll often misclick when selecting your squad members.
• Tower hit points are often obscured by terrain.
• Excessive gibbing causes occasional slowdowns.
• Using the Engineer is basically a requirement since only he can repair on demand.
• Rocket-guy feels by far the most powerful.
• With two squad members used almost by default, the third one will generally be the tank/healer since only he can heal others.
• Poor balance between levels and a few times in levels.
• Sometimes it is unclear whether or not towers can target something over/through terrain.
• You're required to memorise where to find each tower since the UI withholds this information.
• Excessive grinding after about 10 hours/15 levels.

Conclusion

While easily being one of the better Tower Defense games on Steam, its poor balance and especially the unnecessarily tedious grinding end up crippling it. OTTTD would have benefited greatly from a more balanced repair system and cheaper squad equipment that precludes grinding.