In my experience, their prices are competitive with other on-line amateur radio dealers, they have really expanded the product lines that they carry, and shipping is outstanding.
Their warehouse is co-located with the Summit Racing warehouse. Summit is a major retailer of auto racing and hot rod parts and DX Engineering has been able to leverage Summit's established logistics partnerships. There are literally UPS semis backed up to the warehouse every day. If you get your order in before 10 pm Eastern Time and you asked for ground shipping, it will go out that day. Granted, your new antenna tuner may go out in a truck next to some other guy's new camshaft for his hot rod, but UPS doesn't care. A box is a box. And, shipping is free on most orders over $99.
When it comes to building antennas, DX Engineering has some unique products that you won't be able to find anywhere else.
They run sales all the time, though their prices are already pretty fair, so the sale discounts are often not that big.
I've ordered several items from them and highly recommend them.