I don't have the 70-300mm DO, but have shot quite a bit with another photographer who uses one. She gets very good shots with it... mostly equestrian sports, so the autofocus is up to even the most challenging tasks... always handheld, too, so the IS works well.
But, bought new it's quite expensive. In fact it's even a little more than the 70-300mm "L".
The main advantage of the DO is it's smaller size. It's about a 1.5 inches shorter than Canon's other 70-300s... when fully retracted. That might be nice for travel.
But, except for the "L", it's not any lighter weight than the other 70-300s. All three of them come in about 1.5 lb. (The "L" is about 3/4 lb. heavier, without the optional tripod ring... probably a full lb. heavier with the ring.)
The DO is also one third stop slower than the other three at the 70mm setting. It's an f/4.5-5.6 lens, while the other three are f/4-5.6.
If I were spending what these lenses cost new, I'd opt for the "L" instead, primarily because it can be fitted with a tripod mounting ring. It probably has better sealing for dust and moisture resistance, too.
$500 or less for the DO sounds tempting.... But also is a huge amount of depreciation for a Canon OEM lens that's still in production. It's a loss of almost 65% of value from the purchase price when new ($1400). In comparison, the 70-300mm "L" sells for $1350 new and around $1100 used.... Depreciation of 20% or less is a lot more typical of Canon OEM lenses. Heck, even the now-discontinued 70-300mm IS USM is seeing less depreciation, about 38% ($450 new, $280 used).
The new 70-300mm II with 4-stop IS, "Nano" USM and that spiffy LCD distance scale is selling for $450 now.
You might read the buyer reviews for the various lenses. The "DO" obviously hasn't been a big seller, introduced in 2004 it's got 75 buyer reviews at B&H Photo. Introduced a year later in 2005, the less expensive 70-300 IS USM has over 900 reviews and, intro'd 6 years later and priced similarly, the "L" has 367 reviews. Heck even the newest 70-300mm "II" IS Nano USM has over 30 reviews in less than a year.
So, I'd call it a mixed bag. The "DO" is quite capable, but also the oldest 70-300mm model Canon is offering today. The used prices are attractive and maybe it's depreciated as far as it ever will. But it's still slightly more than the latest and greatest "mid grade" model.... and it's not built or sealed like the "L", which also is the only Canon 70-300mm that can be fitted with a tripod rind.