Ok, I am currently playing with the Germans.
This are my early game observations:

Strengths and weaknesses:
The Germans seem to be the strongest barbarian Faction.
While the Gallians seem to suck in general the British tech tree requires to much before it can rival the strength of the German Phalanx.
To put it short: No Level 1 unit in the vicinity can beat the German Spears, and even most Level 2s will strife.

German have also very fast acces to an interesting early flanking unit (screaming women).

The Skrimishers are ordinary as is their Light Cavalry.

Their heavy infantry is also oridnanry, they are better than the Gaul counterparts but they do not really reach the brillaince of the German spearman.


Your early battleiline should be composed of your general, 2-4 Spear units, some Light Cav and Screaming women or Axemen.
If you get Archers use them, German Archers are important because they can be used to force your enemy into your hands.

Strategy:
Germany is poor. The first thing I doo is bringng trade and famring income online by builing a road, a level 1 famr and a Merchant in every city.
recruit some peasants to serve as preliminary Garrisons.
Every peasant you get nets you one big angry german Spearman who can be used for rapid early game expansion.
Where to expand?
Well, the Brittains always attack you (their goal seems to be Trier or Begicum or both). put your spears together and kick them around a bit.
The taking of Saxony, should be also clear.
I would recomand to expand into the east and the south.
In detail, try to aquire modern day prussia, modern day Lithunia and possibly Bohemia from the rebels.
If the Dacians beat you to Bohemia skip it.
I have made the experience that it is also possible to get as far a Russia.
You could also take Iovacium.
I would adovocate agasint taking Lugdugmum, this will always lead to a war with the Gauls. use your spy to check the istuation in Italy, once the romans have taken Mediolanum and modern Venice, engage the Gauls.
Try allying with the Juli, bolster this up by allying with the other romans too.
Chances are that the Juli will go for Spainor Karthage.
Now comes a buildup phase.
You may try to sack Great Brittain, however it is usually enough to sink their navy.
Basically, start preparing for the inevitable crush with rome.