| mmmh... I think the general concensus is yes.
For me its all about confindence. Think about the last TD you did, and how you gained confidence (better lines/corner speed etc) during the course of the session. Obviously a lot of that is due to getting used to the circuit etc, but a lot is also down to the bike reaching its 'working temp' as the session progresses. Everything from tyres/oil in the suspension/engine/rider reaches its optimum temperature...and the bike just seems better allround...less nervous...smoother...leading to better laps. Anything I can do to get to that stage, quicker, is worth it in my opinion!
This will only be my second year of trackdays (fast novice/slow inter)...and my main worry when lapping is losing the front or rear! Personally speaking, knowing the tyres have at least 'some' heat in them prior to going out will make a HUGE difference to my confidence.
As for sitting in the pit lane...from what I've seen, most guys who have warmers seem to leave them on for as long as possible, and then join the back of the que. Obviously if you are sat there for a while (due to an accident or summut) then yeah...your going to lose some of the benifits...but I think for the money (£130)...they are well worth the investment if you do more than 2 or 3 TD's a year.  | 
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