"zɛn" 正can be used in most situation in our spoken language ( not in formal written ).
It can be used to describe anything, food, people, event, etc. But avoid using it to describe people as it is not polite in some situation. With people, it's better to use more precise and elegant word. Of course, if the listener is your good friend, it doesn't matter:).
using such a "strong word" on people may cause confusion in some case.
For example, if some people say "That person is really 正". That implies that person is either really good or the opposite, extremely bad. You will need more information to understand the real meaning.
正啦！("zɛn - la") is a little different. The 啦 actually make the feeling soft. If you want to say something REALLY great, and show your strong feeling, you can just say "正！“.("zɛn"). = great!
Some people say"正呀！“.("zɛn - a") ， it gives a even stronger feeling than the single word "正！“.("zɛn") .
For your information, 啦 is actually a magic word in Cantonese and it have multiple usages. Usually, it modifies the previous word with a soft feeling, except that you speak it really loud.
啦, For example.
1. to ask / force / persuade / request / induce someone to do something. ( depend on your tone, it gives different meaning )
2. to accept someone's request, but reluctantly / doing as a favor / being friend. ( depend on your tone, and relationship with the listener, it gives different meaning )
3. to show something is not 100% perfect, but nearly perfect.
4. to refuse someone's suggestion softly, with less hard feeling.
5. to show it has been finished ( equivalent to 了 in the formal written form）
6. In making comparison, you show your choice is relatively good, or the best choice to choose from.
It looks complicated, but you can just listen to other people's conversation with different tones, you can know the differences.
I hope this answer your question, it's a little bit complicated to describe by words:)