Pros: He was a very important piece in the victory of the red army in the civil war. His writings on fascism are IMO some of the best and most concise descriptions of the rise of fascism. Permanent revolution, while I tend to disagree with a lot of the premises, is a pretty interesting concept that I think any Marxist should take into account.

Cons: I'm not going to get into any mud slinging about him working with fascists since there's not a ton of evidence there, but it doesn't really matter. In the end, his polemics against the USSR after his exile aided the fascists and materially that's all that matters. He was pretty demonstrably wrong when he was part of the worker's opposition. He was also, by all accounts, just a pompous douche who refused to change his mind even when presented with new evidence.

For further readings by Trotsky, I would recommend What Fascism is and How to Fight It and The Permanent Revolution, Results and Prospects. Unfortunately due to his divisiveness there's not really any remotely unbiased biographies of him, but Robert Service probably has the most in-depth work on his life.

I'm by no means an expert here, so anyone can feel free to check me on anything I wrote.

Edit: This reminded me of my favorite Trotsky quote actually, from Hitler's National Socialism:

Not every exasperated petty bourgeois could have become Hitler, but a particle of Hitler is lodged in every exasperated petty bourgeois.