Hi Eyal,
I've used the public API and annotations for Java and they work quite well in most situations.
The only issue for myself is that annotations could still be improved as I've discussed in a different thread here: Elastic APM Java Annotation API Felix provided a workaround but we can't use that workaround all the time; still better than nothing 
One thing I couldn't find a reference to is if it is possible to manually set the traceID and/or transactionID of a span or trace. The use case for this is to be able to propagate IDs across services that do not yet have Elastic APM agents.
Let us say we have a Java service, we can get the current transactionID and embed it in the message payload as a key-value pair, pass the message to a legacy Perl service, which process it and then passes it to another Java service. The second Java service then sets the parent ID to the embedded transaction ID.
I've looked at the OpenTracing bridge but I'm not sure it can really be used in this case.
Thanks,
Ronald