I have a 99 Santa Cruz Heckler, I'm thinking of changing the frame for a Specialized Enduro frame ('04). Can anyone tell me if this will be an improvement. Or any other suggestions, I want something that you can ride all day and take on some rough stuff to? Are any of the other spesh FSR frames worth looking at?

Cheers

Depends on your budget. 

I've got an 04 Enduro and I'm really pleased with it, it's versatile and an excellent choice for all day riding.  I've seen Enduros tipping the scales at 29lb fully built.

The only thing to watch for is the non standard sized shock, at 7.625" you'll struggle to find a replacement.  However, Romic make an excellent coil shock to fit (which I have) if you prefer a coil shock to an air shock. 

I'd invest in the little duckbill mudflap that protects the shock hardware too, these usually retail for £8 from Specialized and keep the crud away from the shock bushings.