In 90% of the games I play with a defensive team-mate, the flak was quickly taken by 4 AB squads and our defeat secured. I realize they are deadly in the right hands, but they are still extremely vulnerable to things like AB, 57mms, and howis. In my experience, all you need to do is stay out of range and pummel it with indirect, or swarm in with infantry and it's yours.
The 88 is expensive, requires too much effort/attention to be very useful, and is very situational.
In the right hands and in the right situation, it can work wonders, but those ideal conditions are so rare it's not worth it, when other alternatives are more reliable, and even a better form of AT, often times.
Also, it's in no way worth a T2.