I'll second the PBEM aspect of this game. Loads of fun.
It's takes a while to learn the user interface, but the rest of the game feels intuitive. Units, terrain, supply, HQs all feel right. The interface gives you every thing you need and I think is done very well. It just takes some time to figure it out.
Tanks attacking infranty in open terrain (e.g. plains and deserts) will do very well, but far less well against infantry in woods, mountains and cities.
These terrain types give large entrenchment benefits, especially to infantry, who entrench quickly. Woods, etc. also give attack penalties to tanks.
Artillery, planes and other unit types (e.g. flak, anti-tank guns, etc.) are also handled well.
Supply and resources are one of the best aspects. If your units can't trace a line, devoid of enemy units or hexes, to a city providing supply, they will run out of supplies and will not be able to move (eventually) and will be easy targets.
If you don't control enough ore mines, you can't build tanks, planes and artillery.
If you don't control enough oil wells, you can't move or attack with your tanks, planes and trucks.
I just had one guy invade behind my lines, cut my supply rail line and almost cut off my invading army. I've heard others have tried the same trick with paratroopers.
A truly fun game. Also, the community and players are very cool.
Ask lots of questions. It took me a while to figure out the basic concepts. After you figure out the interface and how to produce unis, move, attack and keep everyone in supply, I think you'rll really like it.
Oh, and there is research as well. Should I go for Medium Tanks (e.g. shermans or panthers) or should go for the next generation riflemen, artillery or fighters (and many others)?
As you can probably tell, I really like this game :)