I bought my first Warwick Thumb a few weeks ago and researched as much as poss as a nice one isn't cheap and unfortunately you need to travel around to get your hands on one. Loads of good advice already on here but if it helps:
Definitely play one before buying it if you can. Before I bought mine I played a mix of 4/ 5 strings, bolt ons/neck thrus, new/secondhand and they were all very different. Admittedly it wasn’t set up but playing a brand new bolt on at £1500 was completely uninspiring – a 20 year old bolt on 5-string at £750 was stunning both in sound (possibly due to different PUP position) and fast playability.
I ended up buying a 2004 4-string neck thru - different league in the looks dept. The bubinga pomelle finish, thru neck and ebony fretboard make it a sheer pleasure to play. I’ve owned my bass for about 5 weeks now and I cannot put it down. I even sometimes take one last look at it before I go to bed (I KNOW I shouldn’t admit that ).
The weight is heavy but emphasises the cracking build quality. There is great depth to the high notes - previous basses I’ve played have been tinny when playing notes on the D and G strings – not on a Thumb.
The neck IS long and can take a bit of getting used to. If you're trying one out play it standing up – completely different experience to when you’re playing it on your knee.
There’s a good link on you tube which gives a nice demo on the sound options – all I can add is be sure the Thumb sound suits the band you play in as it's distinctive and may prove challenging (as it currently is for me playing in a rock covers band):
[media]http://www.youtube.com/watch?v=TOLxxevOCzU&list=FLmdpxVZClKWdeg_xgw0ggqw&index=12&feature=plpp_video[/media]
Sorry to ramble on. Hope it helps. If you're ever passing through Swindon you are welcome to drop in and have a play on mine.