For me the G25 is well worth the money, I have it combined with the gameracer "Corbeau" Pro racing seat and it is fantastic.
The Force Feedback is when for example you start to loose front wheel traction on corners, you feel the wheel slip / go light which tells you that you are on the very edge of adhesion and would best not push it anymore. This feedback gives an edge over the sixaxes as you only know from the loss of controll with the standard controller that you are approaching and passed the edge of available grip.
For me, the Manual Gated shifter is the only real downside, it is far too loose to move between the gears, It needs tightening up in my opinion. there is no feedback or feel when moving between the gears, and it needs to give you some feel of being central so you know forward is 3rd, rather than slipping into 1st or 5th in error. so for manual gears, I always use the wheel paddels which is better as it does not require clutch either, which for me is too much to handle at the moment whilst remaining comeptitive.
If your serious about your racing, you need a wheel, if you are in a position to do so, get the G25, and if you can go the extra 500 GBP, get a proper racing seat to mount it, then all you need is a 50+ " HD Bravia and you are all set.
GT has cost me well over 3,000 GBP so far (PS3, Wheel, Seat, TV, Surround all purchased exclusively for GT), but im well happy with it.