I'm in the minority here, but I just can't, for the life of me, figure out why anyone would ever bother spending so much money on a brew kettle. What do you get for the extra money? Let's break it down.
1) A built in thermometer - Here's a tip for you, water/wort boils at 212F ;) Seriously though, if you NEED a thermometer because you're monitoring the temps during chilling (or whatever) a decent probe thermometer will cost you $20. Plus, you can move the probe around to ensure you aren't in a single hot/cold spot.
2) Ball Value - This is actually pretty nice, but is it "$300 nice"? Furthermore, you can purchase a weldless valve kit and make your own valve in an aluminum pot. Or pay someone a couple bucks to do it. Or, you can live with a $15 Auto-Syphon.
3) Stainless Steel construction - Very nice, super shiny, fairly expensive. The myth about aluminum and Alzheimers has been disproven, so I'm not sure what the extra benefit would be.
4) A sight glass - For volume estimation. You can replicate this by marking some lines on your mash paddle for your pot for the different gallon levels. Or, you can open up that dusty old geometry book and figure out the volume of a cylinder (Pi * radius squared * height).
As stated in the comments, this is going to be your opinion ultimately, but you should break down the features you'll get from the nice kettle, and decide if its worth it compared to the baseline price of about $30 for a 10 gallon aluminum "Tamale Steamer". Its what I use, and is available in most Latin grocery stores. A 10-fold price increase for me to go from my beloved steamer to a "real" kettle like the one you linked to is FAR to excessive for me to be considering it at the moment.