Well in practice so far, concerning in melee, I've moved a Testudo into the center of 4 charging attackers, and interesting enough they held all 4 for a very long time til I got bored of watching them hold and slowly die. But if anything Testudo's cannot be flanked that is 1, and they can fight all directions while taking arrow fire and not be swayed. Also you can tire out the enemy using just 1 testudo, 1 per 4-5 groups of enemy , not including archers and that Testudo will stand and deliver while the rest of your army watches.

After I lost 50% of my Testudo and I had the enemy starting to tire, I charged the rest of my army and suddenly every single unit on the other side of 10-12 routed and fled.

That's my single real memorable experience which is why I was asking in the first place :) From my understanding its like a turtle with spikes on all directions so that you take a lot less damage and can wear away at any opponent that dares close in on your Testudo ( does not include Elephants -_- , or chariots with blades coming out of their wheels " dunno " ).