Upgrading - Just Do It

I've seen a lot of good advice here that I have either used before or are going to use, but I haven't seen much on upgrades.

Yes, upgrades are the most important thing in this game, especially against the AI. Those engineering bays and armories are not just stepping stones for more advanced buildings, they allow you to upgrade your little weakling warriors into full-fledged destruction squadrons.

Case in point: siege tanks. While being a very powerful unit in the first place, only by upgrading at the armory does it really lay waste to everything. Once you get the chance to build an armory, build one...and then another...and maybe even a third. Not crazy in the least bit because while you're upgrading your guns from level 1 to 2, you can upgrade your ships' guns from 2 to 3 as well as their armor at the same time. Once your siege tanks are up to 85 (70 + 15) damage points in siege mode, crank out about 4, pair 'em with 4 Goliaths for infantry, and some SCVs for repair, and no one can stop you! I've used this combination tons of times and it works great. But you have to upgrade.

Other case in point: bunkers. Want know why your bunkers are getting wasted when the enemy rushes you? Because your four Marines are blasting' away with level 1 guns! Use that engineering bay to upgrade them to level 3 and watch the blood splatter.