Is GRANDPA warp sync the recommended method for syncing a node?

Is warp faster than fast sync?

Yes, it is faster. Warp sync only verifies the Era changes from genesis to the tip of the chain. When reaching the tip of the chain, it will download the state for this block and after that the node will sync normally. In the background the node will then download all previous blocks to store them. Fast sync downloads all blocks from genesis to the tip of the chain without executing them. When reaching the tip of the chain the node will download the state for this block.

Both are doing similar things, but warp sync will get you faster a working node and it is also more secure as you verify the era changes.

Is there ever a use case where I should use fast (or even full) instead of warp sync or is warp sync the better and recommended method for syncing a node?

Currently warp sync in Substrate doesn't work for nodes with a db that already contains some blocks. However, in general you should always use warp sync. There are also some issues in the Substrate issue tracker to make warp sync the default syncing mode. You will need to use full sync mode if you want to sync an archive node.

If I understand correctly, warp sync does not check for transaction validity and does not need to because it uses justifications/finalization proofs.

Yes, warp sync only verifies the headers of the era changes.