Positives:
It reduces workload.
Generally speaking, the shared planning approach lowers the weekly workload as it becomes rare that you prepare every aspect of a lesson entirely by yourself. Doing everything yourself can result in a very heavy and, sometimes, unsustainable workload.
It allows for the use of expertise that others might lack.
If you, or one of the other teachers in the shared planning team, have a particular set of skills or specialist knowledge, it can be built into the planning process and benefits all classes rather than just one.
It allows for direct comparisons
You can do a lot more moderation between classes because the children are working on the same things in the same way. That may make assessment easier and quicker.
It increases the perception of balance between classes.
For some parents, this is important. They don't want to feel that they got the raw deal because their child is in X's class rather than Y's class. The use of shared planning (with parents knowing about it) decreases the likelihood of that.
Negatives:
You are constantly drawing on someone else's work
This can have a lot of implications. You may find yourself waiting for resources because the person hasn't made them yet. You may find yourself constantly having to make up for the lower skills/knowledge of others in the team.
Things often need overpreparing
Anything you need for the lesson (plans, resources, slides, examples, etc.) need to be prepared in enough detail that someone who hasn't been involved in creating it can just go teach it. Sometimes, this actually makes the planning take longer.
Limited scope to tailor things to your style or class
There could be significant differences between the classes involved, in terms of levels of need, academic ability, behaviour, etc. If that's the case, the prepared lessons may not work for you. You'd either have to adapt them heavily (if you are allowed to) or, if not, ignore some of the specific differences in your class.
Over time, it can deskill teachers
Overly drawing on things created by others can have the same effect as heavily using schemes. Over time, it erodes the confidence and skills of teachers in the areas they haven't specifically planned/resources. This can be particularly true if the same people on the team always prepare the same subjects/topics.
Personally:
I've worked in a range of settings from a single form with nothing to share to a 3 class all shared planning system. For me, the negatives outweigh the positives and I'd much rather be in a single form, and responsible for it all, over having to operate a shared planning system again. But it does depend on the school too. My previous setting was insane with the headteacher saying she expected to be able to walk out of one classroom, into the next, and experience the same lesson being taught in the same way and it even be roughly at the same point. That approach helped no one, least of all the kids.