It's getting increasingly grim - if you've got a decent full-time post right now I'd hang onto it to be honest - with TAs/HLTAs/Cover Supervisors starting to kick into primary a lot more covering classes - the work's getting thinner on the ground, especially if you want to be picky about what you take (I am!). Once you get known to an agency (no LEA lists anymore around lots of places) and build up a bit of a history of working well in schools it gets easier - you get repeat bookings so you learn faces and names etc, but that takes time, and the staff turnover in some agencies is such that you're always faced with the possibility that the consultant you get on well with may move on and you'll get another who, to put it bluntly, just doesn't like you - I've been through it, it hammered my work down to the ground and thankfully the one who didn't like me moved to a different desk fairly quickly!
Pros:
I leave problems at the school gates, if it's a cruddy day in a grot school - I never have to go back there! I get the fun stuff of being in front of a class teaching, without the extra stuff that drags the life out of you. I get endless variety - can be chalking numbers on the playground with reception one day, and learning about India at the other end of the school the next (or even one in the morning, one in the afternoon). Once you get a group of schools you tend to bounce between - it gets really nice as you know the kids (but without all the paperwork) and you've got some status as being a regular face in the school.
Cons:
Not knowing where the loos are and the politics of staffroom chairs/coffee! It can be quite a lonely existence - there are days where no one apart from the kids calls you by a name other than "the supply".
Morning calls - I hate them - I don't do them, the agencies know by now that I'm utterly shite at them and liable to ignore my phone at 7am - they try to give me the option, but it's taken a long while for them to get to the level with me where they'll accept that. I'm picky about ages/areas I'll work at - it cuts down on the work I get (but Y6 post-Sats are NOT fun to do supply cover with!), but I'd rather do the stuff I do well, than stuff I'm not as good at.
Behaviour - you're constantly having to push harder with class management to resist the testing of boundaries that they get out of the way the start of September with their full-time teacher.
Mind-reading... apparently this is something you acquire the day you start on supply. You can get negative feedback for the most ridiculous reasons (my most noteable one was not knowing that reception had to do PE in utter silence... how was I meant to know that telepathically?). You often don't get told that assembly, despite being 9.10 on the timetable, is actually at 11.03 am on the second Monday in the thirteenth month of the year - or whatever... you're just expected to KNOW!
September... it's dire, it never isn't dire - budget accordingly. July's a bit of a dead loss as well. I mark SATs and tutor - it keeps us partially afloat but we do rely on my husband's income a lot. You CAN sign on for contribution-based JSA during the summer - but it's usually a collossal pain in the rear getting them to accept the claim (see the TES supply forum if you're wanting the low-down on that can of worms - about the start of July is generally a good time to find about 30 threads on it).
The incredibly stalking lesson. That one that every school in an age group is doing on a particular week - Katie Morag I'm looking at YOU! - you end up teaching it five times, to five classes over the course of the week. PE and Music have the peculiar habit of stalking me in a similar way too - as has yard duty!
Agencies - good, bad and downright awful. From the not taking CRB portability saga that means I get landed with the best part of a hundred-quid bill for them all expiring at once! You can get agencies and consultants that are very very arm-twisty about you doing stuff that you're not comfortable with, who ignore your requests for work preferences - and you can get some utter gems. There are some right bullshit merchants out there though and if you're in the game for any length of time you see the same fibs repeating themselves - the phonecall in the second week of July telling me they were "about to get really busy" had me nearly wetting myself with laughter!
Oh and add in that you'll be blamed for everything, that quite often full-time teachers seem to rabidly hate you (I love the "overpaid crossword addict" insult someone threw at supplies), you get schools that seem to deliberately TRY to trip you up in order to complain about you to the agency, and the odd double/cancelled-booking wild goose chase.
I'd look into it very carefully these days - I love doing supply, but it seems to be getting more financially perilous each year, and I lose more and more of my regular schools to getting in-house on the cheap cover.
If you're doing it - buy a sat-nav... seriously - it beats the 7.30am magical mystery tour with an A-Z balanced on your knees no end!