talk about impact of "alter table .. shrink space"

I have read so many wonderful things about "alter table .. shrink space".

However I still hestiate to use it on our large tables since we cannot afford full table lock on the table and behind cost on the system.

In our OLTP system, the table could have billion rows, and could have over 20 transcations per second.

Can someone explain to me what kind of impact this could cause, such as on CPU, redo logs, and UNDO?

I read that the full table lock only happen at end and only for very short moment. My concern is if at same time, there are people doing insert/update, what could happend? Is it could cause any dead-lock?