Watching the first season is a bit of a chore but Arrow significantly improves over time. It lacked direction at the start because there wasn't much of a story arc, then the episodes took on a more serial element with recurring characters, stories and it gradually got darker too.
Flash is pretty much the same show (Arrow spin-off, same writers) just a little more light-hearted. On it's own it's probably not great but it's a fun add-on that Arrow fans should enjoy.
If you do give them both a go, you can't start Flash until you've seen the first 2 seasons of Arrow, because season 3 ties in and exists at the same time as Flash begins, and there are crossovers.