I've been several times. They're gorgeous.
I haven't camped but husband has lots of times as a kid and we're considering it for next year but in one of the bell tents if we can.
We always go to Bryher. The campsite is very basic but has everything you need. I know the other islands, except Tresco have campsites too but I've not been to them. St Agnes is meant to be a nice campsite but that's the only island I've not been too.
You need to book early as they fill up quickly. They usually have a date that they start taking booking, but they'll open bookings up to regulars a week earlier. At least that's what happens on Bryher.
We're hoping to get a bell tent on Bryher as we take so much stuff when we camp that I can't get my heard around the logistics of transporting it all yet. We either need to downsize, or get proper containers for all the things I just chuck in the boot normally... like the stove, pans etc. I need to see how other people do it.
What I do know is you load all your stuff into containers on the quay at Penzance, there's a container for each island so you just have to put things in the correct container. If I remember rightly gas bottles go in a separate container, so you need to clearly label everything. Once you've loaded your stuff you then have to park your car, there are a couple of places specifically in business for that and they'll drive you back to the quay to get on the boat. I highly recommend Scilly Parking, they were amazing with us last time and rescued us twice due to a flat battery (left a light on in the car for a week!) They'll take normal luggage, but not camping so you have to load your camping stuff at the quay first.
Then you get on the Scillonian and the containers are loaded. Stock up on sea bands and qualms, even a "good" crossing makes people green. It's a flat bottom boat crossing where the Irish sea, English Channel and Atlantic meet. They can get a bit argumentative!
If you're going to St Mary's you'll collect your things from the container on the St Mary's quay and I would guess the campsites have a landrover or something to come and meet you and get your stuff to the campsite (probably at a small charge)
If you're going to an off island, the boats pull up alongside the Scillonian and your things are thrown from the Scillonian to the island boats. That's the bit that makes me tetchy, along with the logistics of packing it all. Once the boats are loaded the passengers get on and you're all taken to the off islands. There's a fee for the inter-island ferries too. Once you're at the island your things are passed onto the quay (usually everyone joins in with a human chain) and someone from the campsite should be there to get your stuff up to the campsite. On Bryher it's a quad bike with trailer. You load your stuff onto the trailer then you have a short walk up to the campsite.
They are amazing, and all the hassle for us of a 10+ hour journey, plus an overnight stay the night before and one somewhere on the way home is so worth it! The crossing can be difficult, but it makes you slow down, breathe in the fresh air on deck (to stop you feeling sick) and prepare for a slow slow week of beautiful huge white sandy beaches.... with no-one else on them, no crime, no one locks doors there and having to walk everywhere, but when you're on an island 1 mile long by 1/2 a mile wide walking everywhere isn't a problem.
I could go on, but won't but feel free to ask questions. It's my favourite subject :)
Oh and it's the Isles of Scilly, some people get very annoyed when they're called Scilly Isles. ;)