I'ma tell you what my new favorite toy is...the universal gadget! =D
Now I've really started to like this item and I'll tell you just why.
1. Deals 650 damage to enemy.
2. Heals yourself for 2000
3. Heals your towers for 2000
4. Heals teamates for 2000
I'll start with 1...this is one of my favorite features, because whenever I'm beatin down the enemy and he starts to skidattle, maybe I can't keep up with him or he's getting really close to a buncha towers, you can seriously surprise his ass with a gadget kill.
Now for healing yourself you could either buy a whoooole different item to heal your self or you can just keep your gadget handy for a convinient heal...and the reason it's so convient is that you can use it for so many different purposes and is soooo much better than a regular old health potion...sure it heals 1000 less but heck, you can heal allies with it which can earn you a couple of great big thanks's
Now 500 gold isn't that expensive and I'll tell you what...from my experience using them so far, it is way worth it. I have saved myself with one, I have saved teamates with one, and I have scored a few kills with a few, not to mention you can keep your towers from being obliterated if need be.
My suggestion would to invest in a few throughout the game.