I really enjoyed it for a couple of years (primary) generally lots of work in my area but still some quiet times (September)
Definitely made me a better and more relaxed teacher and you realise a lot of stuff that seems really important when you're in a school actually isn't that vital (e.g. you're under increasing pressure to implement new complex marking scheme, the school 3 miles down the road doesn't do that at all but they're spending loads of time on new SEN assessment paperwork!
I did some really successful lessons off the cuff and kids didn't suffer at all from lack of success criteria!)
It did make me realise a lot of schools are not great places to be at the moment, which gave me confidence it wasn't my teaching that had gone downhill. It's the system. So depressing, but in a heartening sort of way??
Have a plan for the summer holidays, you'll really feel the pinch then. Extra exam marking in summer term to help you save up or I taught English to foreign students (I'm lucky there's a market for this in my area and i had no childcare to consider)
I couldn't really avoid using agencies but you can shop around for best deal and negotiate higher rates if you're experienced.
I didn't take any contract longer than 4 weeks as then you are expected to plan, deep mark, attend parents evening, write reports etc. If you take a contract longer than 12 weeks they have to pay you at scale point but it wasn't worth the pay off for me. Casual short term means you can leave at the end of the day and not worry about it.
Most schools are really grateful for anyone half way decent. I felt like when I had permanent jobs however much I did, it was never enough. On supply the smallest effort you make (e.g. actually trying to follow marking scheme, not leaving classroom in a state) is very much appreciated. It may depend on what's in your area of course!
Good luck!