Regardless of who anyone buys it from, It has to be quality stuff. You can easily get a 4 pack on ebay but do I trust them in a plane? No way.

Engine dies, no biggie, dead stick and land it. Battery dies/shorts/blowsup well, not much you can do other than watch where the plane crashes so you can recover what you can.

DO NOT SKIMP on the battery packs and this goes to all people looking to get one (I'm guessing this thread will come up in searches a lot, assuming people use the search function).