wokwok,
You look at anything and compare it to anything else and your pretty much going to get mixed opinions. I would guess that a lot of the 'not so good opinions' are from people who, have a friend, or know someone, or have bought or tried the cheapest kit or made their own from scratch, it's like saying "yeah I drove a Yugo, wasn't that great, I'm going to stick with American cars" Though it doesn't take that much money to actually buy the parts for GREAT Studio Monitors, it's how you put it together, for instance, you can buy the same woofers as the Mackie "824" for $30.00/ea., the same tweeters as the Blue Sky "System One" for $80.00/ea.(though they might have theirs slightly modifyed, maybe not), make some crossovers(this is what can make or break any speaker, infact most consumer speakers can be made to sound so, so much better by improving the crossover) for about $50.00/ea. put them in a solid non-reasonant box, and wala for under $350.00 you've got some monitors that sound like they should've cost at least twice that, (I should point out that I've not built this system, I would use a better woofer) but it can be done, the problem is having the know how.
Ive started building speakers back in the mid 80s (right after I got back from "The Recording workshop" in OH), first by trial and error, than I became more serious and got a book from RadioShack, then got more serious and bought "The Loudspeaker Cookbook" and some acoustical measuring equipment (your just building blindly without measurements) I never got into building real electronics ie: pre-amps, power amps... just speakers, to me their the most important component in a system!
So yes, you can build some monitors that are better than you can buy, might not be able to get it under the price of the yorkville, but I can tell you they'd sound a lot better.