A couple reasons.
1.) "Consider the Source." Musicians' Friend is a music equipment retailer, so obviosuly they're going to rave about it - they want to sell the stuff. And Youtube is an unfiltered knowledge database, so you have to take everything you see there with a grain of sand. The Spiders are Line6's entry level line, so they're really targeted to beginners. As such, most of the people raving about it haven't really played anything better. If someone who has a wide range of exposure to different amps, coupled with a fairly open mind, tells me it's awesome, I'll definitely make a point to check it out. If a guy who's been playing 16 months, has plugged into a couple solid state Peaveys and once played a Dual Rec with the gain dimed at bedroom volumes in a Guitar Center raves, I'm not going to be too inclined to listen.
2.) Watching bands play through them. I can't tell you the number of times I've seen guys playing Line6 amps get abolutely buried in the mix. I have no idea why this is the case, but they absolutely don't cut, if they're going head to head with a tube amp (for some reason they do better if you're playing in a 2-guitar band with another Line6 player. It's weird, I don't get it).
3.) Personal experience. I've done some recording with a friend's Spider in the past, gigged with it once (after the TSL100 I was playing at the time got doused in 2 pints of beer 15 minutes before showtime
), and I've demo'd every single Spider so far. They're easily my least favorite modelers - the Roland Cube series stomps all over them, the Vox is a bit more organic, the Johnsons, now out of production, were overall pretty iffy but the Mesa patches were awesome, and Tech-21 doesn't really deserve the modeling tag. Like I said, I was pleasantly surprised by the Spider Valve's clean tone, but the rest of it was just stiff, sterile, unorganic, and unresponsive. They're really just not that exciting amps - there are better modelers, but once you get used to the idea that you don't really NEED 32 different amp sounds on the fly, there are WAY better non-modeling options, even in your price range.
If you really like it, then go for it... But odds are within 2-3 years you'll have outgrown it.