In my experiences, the Panther is best used like a mobile AT gun. You keep it rather far back, with target vehicles only turned on, and wait for shit to start rolling towards it.
As many people have pointed out, the panther isn't that great at what it does and it costs a lot both for the tech getting there and the vehicle itself.
That being said, it's best to go for T3 and keep your T3 units alive.
In an ideal situation, I'll get my T3 armor out, then my call in tank - whether it be a command P4 or the Tiger - then buy the T4 tech in the HQ, then wait a few minutes, then build T4.
The reasoning behind doing this is due to multiple factors:
1) It's almost always better to spend your fuel when you can rather than waiting for something
2) The Panther is a great tank
3) You're hopefully going to win before you build T4
You have a healthy composition of vehicles that is good at tackling all targets: infantry, support weapons, vehicles. Only weakness is concentrated artillery guns, blobs of handheld AT, and bad micro on your part.
One of my favorite late game compositions is using the blitzkrieg commander and having
2x Stug III G
1x P4
1x P4 Command tank
gives you panzer smoke, stuka CAS, some other fun shit. Tac Movement, I think? Anyway, this is a super potent combo. Stugs are great at taking out tanks and will pen shit up to an IS2. That's when you need bigger guns.
The stugs aren't heavily damaged by rocket artillery.
The stugs have a good HP pool; they can take a lot of hits.
They have a good rate of fire.
They are cheap.
If you're 40 minutes into a game, however, and you've got 250 fuel in the bank and you've got the manpower and you've just lost a stug, buy a panther over a stug. The increased popcap may make you shuffle your infantry unit composition a bit but the Panther has some better traits than the Stug including
Higher armor - takes less damage
more HP - lives longer
moves faster - both 1&2
Has a higher penetration value - more likely to damage enemy armor
The Stug, however, has about reloading time that's half that of the panther's so you're firing nearly 2 shots on the Stug for each Panther shot assuming that you're stationary and the target is staying within firing range and within sight.
The panther is better, though, for its mobility. It allows you to, if necessary, flank an enemy tank, specifically one damaged by a teller mine. The Panther, however, is a luxury. You need to have vetted, upgunned infantry, whether it be LMG42s and/or a schreck squad of pgrens, probably 2 machine guns (map dependent) and at least one, possibly two, other teamweapons (mortar/AT gun). The Panther is very expensive, and wasting your fuel on teching in the 32nd minute right before you've lost a stug, p4 cmd tank, and a vetted PaK to an enemy push isn't the best time to tech up. You've got to be conservative with your fuel and stock up so if a loss occurs and you need something on the field immediately, you have that option.
The strongest wehr gets is when all of its units come together. I wouldn't recommend diving any tank into the enemy portions of the map, but if you've got to do it, the panther is a better choice than the Stug. When you've got to flank an enemy tank, or want to be super daring and hit a katyusha, the panther is your better option due to its speed and survivability if its engaged.
http://www.stat.coh2.hu/squad.php?filename=panther_squad_mp
http://www.stat.coh2.hu/squad.php?filename=stug_iii_squad_mp
I hope my rambling helped
