i shudder to think what i've spent at tooltopia. the closest thing to a bad experience was when i bought the gearwrench 1/4 impact master set from then, then cripes had some at 1/2 the price. i tried to get them to refund the difference, but in the end ordered one from cripe and returned it to tooltopia. they did the credit at that point w/o a problem. i wasn't happy about it, but it wasn't completely unreasonable.
tooltopia is almost always the cheapest vendor, although i've had slightly better pricing at times for sunex/genius from dskatools. tooltopia is free shipping over $100, but i've never seen any coupon or discount codes for them - the prices are just overall cheaper (which i MUCH prefer).
northern is a mixed bag for me. i'm guessing like many here, its fun just flipping through the catalog, but many items i've found from tooltopia or amazon more cheaply (or even my local TSC). still, when i've ordered from them, they've been good to deal with.
ahm