I changed my '98 ES330 valve cover gaskets this weekend. I have worked on cars and equipment for many years and found a few surprises that I have not seen in the forums. On a scale of 1 to 10, for me it was a 6. It is not complex like an automatic transmission but you better set aside a day to do it. Here are a few hints if you choose to do it.
1) REMOVE the battery terminal.
1.1) Keep sand and debris out of the engine. Use compressed air to blow away from the valve covers and the plenum BEFORE removing them.
2) Mark the brackets with a Sharpie and remove the hood. Get a friend, or two, to help you. Don't scratch the paint! This makes everything more accessible. Note the bolts are not identical.
3) Use the Sharpie to mark the hoses and connections of those hoses you have to remove. Mark them as you remove them!
4) Do the front bank first. This will require hoses to be unplugged as necessary. The hose assembly if front of the plenum is held to the engine by bolts. This will lay over to the passenger side and get it out of the way.
5) The spark plug seals have tabs on the inside of the valve cover that "keep" them in place. I used pliers to gently bend the tabs back. Now you have to get the seals out. The spark plug seals are pressed into the valve cover and will take some ingenuity to remove. Maybe someone here has a easy way to do it. I carefully folded some of the seal over with a small screw driver and punch. Then, using a large screw driver, drive it out from the top, hitting on the folded area. Installing the valve cover seal and valve cover is elementary.
To do the rear bank, here are a few tips.
1) Unplug the EGR sensor and Remove the throttle body, not the cables. You may have to use some string to keep it out of the way.
2) Two brackets are bolted to the backside of the plenum.
3) Remove the EGR tube that bolts to the cast part. You will not be able to see the lower nuts. Don't drop them. This will help to get the valve cover off.
4) Cover the plenum openings. Free the rear O2 sensor connector from the bracket and unplug the sensor. Its wires are in the wiring harness on the valve cover.
5) The wiring harness that runs across the valve cover is a pain. I removed two nuts on the back (driver's side) of the engine (under the throttle body and center engine) and unclipped it on the passenger side. This, and the EGR tube removed allow the valve cover to clear the cam gears. Keep working gently to clear the obstacles to removing the valve cover.
During reassembly,
6) Before the plenum is tightened down, install the two bracket bolts on the plenum - finger loose. Torque the plenum, then tighten the bracket bolts.
7) Go slow and make sure all hoses go back on their marks and all connectors are plugged in.
Tools -
Retrieval tool to get those things you drop behind the engine
Torque wrench (32 ft-lbs) for the 2 - 14MM nuts and 2 - 8MM socket head cap screws on the plenum.
This by no means covers everything. Be methodical to the point of writing things down and/or taking pictures.
Good luck! If you are successful, go out to eat with the $400 you saved :D