For some reason or other, I always end up playing an engy for at least part of the game. It's usually safe, it's cheap, and it's an extremely effective way to support the team. The only problem is, you have to be a little paranoid of Black Hand stealths and snipers while running around in base. I've gotten somewhat experienced at this, so I want to share a few tricks and experiences, feel free to contribute an engy/counter-engy tactic. But this isn't the place to rant about running over 4 engineers at once in a Mammoth, find another place where it matters.

A couple of basic rules for the current noobs and future technician elites:

Always repair at the building's MCT.
Never leave an unmanned vehicle, especially if you're on GDI.
If there's buildings taking damage or beacons being planted, check it out ASAP.
And never leave a part of your base uncovered. Use mines to remedy this.

I've gotten several kills with the pistol alone, and my Golden Rule of Engineeringtm is to always load my pistol before anything else. When a Black Hand or opposing engy rushes the base, all I have to do is whip out the pistol and shoot them in the head. The thing that seperates good engies from great engies is if they can hold their own with a pistol. I've shot many an unsuspecting rusher this way, they expect you to be easily frightened off and weak, so I try to send them for the quick and dirty ticket back home. Just go for the jugular and that suprised Black Hand beacon planter should go down pretty fast. If they're any good except as a rusher they'll fight back and you may not have a chance, but you can hurt them and the tables are usually against them living long, if they're in your base anyway. Make a racket and reinforcements should arrive. On a side note, rather than running around as an engy-tool monkey all the time I carry a pistol ready, it's quite simple to damage or kill an oncoming soldier with a few headshots. Repairs can wait sometimes, and you should do all you can to defend and rack up points... I've gotten so many, "WTF!"s from that.

Also, always pay attention to the building damage indicators. If you see a building with 3/4 health or that's just 'Under Attack', it could be a stray shell or an enemy planter. Never assume, but always check it out and follow your instincts and engy-intuition. I've done that many of times and helped save a building, or gotten busy and assumed it's nothing the soldiers couldn't handle... and been wrong. Ka-Boom. And if you've got 3 guys fixing up a building that's being shelled, always check out the other ones, as that often seems to be a coincidental diversion, and no engies are around to stop 3 blocks of timed C4 on the other side of the base. If it's bad in one place, it's gonna get worse somewhere else.

Thirdly, never leave a part of the base uncovered or unsurveyed for long. Put a few mines behind the doors or in the tunnels to stop rushers dead in their tracks. This goes hand in hand with part2, but you can prevent it even happening. If you want, put some mines in any tunnels or side entrances to get even more rushers, but you won't have as many mines to guard the buildings. It's risky... I always try to put mines behind the doorways in groups of three or four, which kills the Engineers and Black Hands quickly. Many times in the last couple of games, a humvee revealed a Hotwire which made straight for the power plant... and got blown up soon as the doors opened. However, this doesn't stop people from planting Nuke or IC beacons outside the buildings, where they can hide better. I wish the programmers would change it so beacons outside the buildings only heavily damage them to 25-30\%, but the rushers wouldn't have it that way...

Also, if you go on the offensive with vehicles and infantry, your repair gun is your only real asset to the assault. Just use vehicles and terrain for cover to avoid snipers and repair behind cover, and dodge all infantry you can. Look inconspicuous and you will live much longer. You also have a crucial role. You can prevent the halfway-rush syndrome that is more crippling to the attack than a Mammoth tank... When people get their nice expensive Stealth Tank damaged, for some reason they turn back around, blocking the oncoming invasion as well, and head back for base because their fricking paint got scratched. If you have repairs on the field, they'll last much longer and won't have to go all the way back home to be total pussies.
You also have a variety of explosives to finish off structures or vehicles that the wusses might not finish otherwise. Maybe they'll actually consider blowing up the buildings and overruning the remaining weaklings. But if it fails and you're being overrun, provided none of the aforementioned wimps give you a ride home, you'd better be fast and good with a pistol if you wanna make the trip back home. Just remember to weigh your choices and choose the course of action wisely, one engy could be the death of them...

Maps that are good for engies:
Without Power Plant, with base defenses.
With few side entrances, defendable side entrances, or highly visible approaches. No back ways around.
Maps with compact base layout, espcecially with base defenses mixed in, and high-traffic zones for the soldiers.

Maps that are bad for engies:
Anything without base defense.
Anything with a powerplant.
Spread-out bases, bases with many side entrances, bases with large open spaces and approaches, bases with many sniper overlooks, bases with low-traffic zones around many buildings and especially bases that have jutting out buildings that are difficult to cover.

Any map where you end up as GDI without any other good engies , especially without base defense, large open spaces, sniper overlooks, vehicle rushing, and spread out bases. I can't remember that map though, but I hate it!

Also, the Flame Tank is not so much devastating as very annoying. It packs much damage versus buildings, like C&C TD did, but for some reason they're much better against vehicles than infantry, one flame tank can wipe out a mammoth. It's a nuisance for engies cause it can melt through buildings at a good rate and splash damage anyone inside, but it's very low cones of damage and slow speed means it's not a huge threat to infantry, just damaging and a little scary, I guess. It's feasible to put mines and c4 all over it and get rid of it, though it's best taken care of by anti-tank infantry. I honestly wish they would rebalance it and make it the good ol infantry murderer again. BTW, the flamethrowers and chem sprayers do better damage versus vehicles than infantry. I dunno why, it just sucks and they're used for the completely wrong reasons than before.

Engy story:
Once while I first engy'd in the full game, I went with three others in the weapons factory while shells and flames hammered it. Everything was shaking and exploding, we would heal each other once in a while but kept at it for a couple minutes until it subsided. It was quite funny to be inside, it was sheer chaos with the exlosions and all. The life bar would creep down, then up, causing blackouts and sparks to be frequent. That was a good time.