For SPS, I think its a mandatory piece of equipment. A lot of people don't have them and rely on fans and what not, but when paying $100 an inch for a piece of LE.......a chiller is money well spent.
I was lucky enough to have a buddy in SF that bought a tank that already had one. When I visited the next time and it was in the closet, it came home with me.......Its a cheap Via Aqua Polar Bear, but has been flawless.
http://www.saltwatersolution.com/store/cart.php?target=product&product_id=17107
I would not go with a drop in and go with an inline unit.......more efficient IMO.
I would get the biggest you can afford, just in case of the dreaded tank upgrade, but more importantly a larger one will have more cooling capacity and will cool the water down faster allowing you to hold the temp at near stable, but additionaly not have it running all day just to keep up.......in the long run, should last much longer than a smaller one due to wear and tear.
Make sure its compatible with a controller if you ever want one or have one.
I was considering one of theseCurrent Prime inline towers:
http://www.marinedepot.com/aquarium_chillers_current_usa_prime_inline_drop-in.asp?CartId= for the nex tank, but thats a long way off...........maybe?
I like that its modular and can be upgraded and has the integral heater also.