Building a 540 with a blower. How critical is a blower intercooler ? I've seen people run with and without them. any advice ?

Having an intercooler will allow you to run higher boost meaning more HP due to lower intake temp.Be carefull if buying a used one .I'm afraid of leaks and over powering my drives ,but if you need more power it's the way to go.

I agree with what Ray is saying,be careful if buying used but it is the way to go.Good luck.

RTech performance wrote this, kinda impressive.

HOW DOES IT WORK?

Every engine builder knows the importance of good cylinder heads. But on a supercharged engine, the intake pathway from the supercharger to the engine is just as important to the overall flow rate and therefore to the power of the engine.

Rtech Performance has researched and developed the low restriction intercooler configuration over the last two years to the point where we outperform typical intercoolers by up to 40% on the flow bench.
New 800 HP
Engine/Drive Package
When typical intercoolers are replaced with Rtech Supercoolers, power increases of 13 - 20% have been achieved, without spinning the blower any faster. Charge air temperatures have been observed to drop 25 - 30 degrees. Dyno-tested gains of 85 - 100 HP over baseline.

WHAT DOES IT MEAN?

Power is gained by increasing flow and cooling efficiency, not by spinning the blower faster. This avoids unnecessary heat and reliability problems.
http://www.rtechperformance.com/about.html

I've run 572 blower motors for many years and have had good luck with out coolers.Mostly by running low compression, low boost and good parts.Using CNC heads and the right cam ,levels around 900HP are realistic.These numbers shown by M D G Perf on their in house dyno using this combo.Intercooling becomes more important above that.But !!!!! they are known to increase reliability and power on all size engines.Did I mention I'm saving saving up for a pair.