Haven't posted here in a while, but just found this topic and thought I could add some usefull comments. We've stayed at Flanagan's a couple of times and really enjoyed it. The big plus for us is the creek (I think it might actually be the upper Logan River???) - it's beautifull, with a rapids section that you can float down on a tyre tube or something similar, which then runs into a nice deep, wide pool. A word of warning, if you take something to float down the rapids on, make sure it's cheap/disposable, as there's a very good chance that at some point it will be punctured by the rocks.
When we were at Flanagan's (Jan and March 08), it was quite dry and the grass was pretty sparce, so there was a bit of dirt and dust around. It was also very busy when we went on Australia Day weekend, although this is the case at most camping sites on long weekends - it was alot quieter the second time we went. These would be our only complaints about Flanagan's. They have showers and toilet's, and a small kiosk. Fire wood can be bought, I think it's around $10 for a wheel barrow load.
We haven't stayed at Bigriggen, but as it's just up the road from Flanagan's we went and had a drive around the site. From the brief look we had, the grounds look nicer than Flanagan's - more grass, less dirt/dust! There is a water hole there, but it didn't look anywhere near as appealing as the creek at Flanagan's, although it might be OK after alot of rain.
Personaly, I'd be happy to go back to Flanagan's - there's been a fair bit of rain recently so hopefully there's a bit more grass. But if you really hate the dirt/dust, it might even be worth staying at Bigriggen for the nicer camp grounds, and just going for a drive up to Flanagan's if you feel like a swim.
I have some pics of Flanagan's, I'll see if I can post them up over the weekend.
EDIT - search for Flanagans Reserve on Youtube and you'll find some footage of the creek.