My current campaign is a pure retro-D&D romp. We started with In Search of the Unknown, added Keep on the Borderlands, Isle of Dread, White Plume Mountain, and then G1-2-3. The PCs have almost finished Hall of the Fire Giant King, and I am about 90% finished with conversions of D1-2-3 and "Kingdom of the Ghouls." (I've also done a conversion of an old White Dwarf adventure, "The Halls of Tizun Thane," as a lark.)
I used Paizo's Isle of Dread and WotC's updated White Plume Mountain; for Keep on the Borderlands I took the conversion in the EnWorld library and updated it to 3.5. For G1-2-3 I used Daniel Collin's 3.0 conversion as a base, updated it to 3.5, and used lower-HD versions of true giants. The other conversions were all me.
My basic conversion philosophy is somewhere between WotC and Necromancer Games. I want to be true to the feel of the original, but not have a strict, slavish aping of the source text. I have freely altered the numbers of monsters and treasures to better fit a 3.x game. So if a "newer" monster better fits the feel or level of the adventure than what was in the original, I will use that instead. If the original text had 16 goblins in a 20 x 20 room, this translates to an EL 5 encounter if you use a strict one-for-one conversion. I would have no problems, though, converting that encounter as 6 goblin warriors 2 -- still EL 5, but more playable and a maybe little more logical.
In general we have had a blast with all these converted modules. We've probably had the most fun with the Giant adventures, but each one gets a little tedious after 4 sessions or so.
Many of these modules I have also run in older editions. And one thing I've noticed . . . the 3e game feels substantially like a 2nd or 1st edition game, but just with more consistent mechanics. I've really been baffled by the complaints of people who think 3.x has completed changed their beloved game beyond recognition into some horrible monstrosity. I don't think 3e is perfect, and there are some older aspects of the game that I miss, but I certainly don't see 3e as all that different in play from older incarnations. In my experience, the line has bred true.