I can only speak for the KR8, because that is what I use. My cousin has a Blu - it's what got me started researching e-cigs in the first place. I went with a KR8 because of the convenience and ease of use. I'm not terribly impressed with Blu for several reasons - low battery life, lack of flavor options, lack of size and color options. I paid more for an all inclusive starter kit (89.50 from Vapor4Life), but I feel it was money well spent.
If I could address a few of your assumptions:
First, the Blu has a pack. The pack holds five cartridges and recharges your battery on the go. Since I'm gone about 11 hours a day, and smoke like a chimney while I'm away from home, I'm going to need something that will last.
It's not just Blu that offers PCCs. I have a PCC for my KR8s, and it works pretty nice. Since the batteries last longer there is less recharging that I need to do anyway! Also, cartomizers will hold more juice than a Blu cart, so you'll need less of them in the long run.
Second, the Blu has disposable cartridges. I never remember to fill my Zippo, and end up having to bum a light about once a week. If I forget to fill my e-cig, I'm stuck at work with no fix. The Blu has room for five spares, so if I run out of one, I have backups.
Again, cartos hold A LOT. I fill my own blank cartomizers, and I can routinely get 30+ drops into one carto. I've never heard of anyone being able to put that much into a Blu cart.
Third, the Blue has easily swapped disposable atomizers. I know damned well I'm going to blow one out at 10 in the morning when I have eight hours left to go before I can get home. At $5 each, I could destroy three a week and still be ahead of what I'm spending on Winstons.
KR8s don't need atomizers - they are built into the cartos. You pay a little more for cartos, but the atomizer is part of the cost, and it's one less thing to have worry about. Easy peasy!
Fourth, it looks like both the Joye 510 and the Kr8 have buttons to activate them, and without a case, I'm pretty sure I'm going to end up pushing that button with the side of my cell phone and burning out the whole thing. No button on the Blu, and even if there was, it's stored in a pack all day.
KR8s come in manual (with button) or automatic (without button). KR8 batts come short, regular, or extra long, in auto or manual, in a rainbow of colors. Blu doesn't. Just two colors, one length, and no option for manual should you want to try one (and you should, many prefer them). Aren't MORE options better than LESS?
Now, I haven't tried a Blu, and have no idea if there's a good hit or if it tastes like .... I can see tons of positive reviews for the Kr8 and the Joye 510, and I know where I can save a few bucks on the Joye. I don't want to drop the 60 clams on the Blu and then end up just going back to buy a Joye 510 anyway.
A lot of people here have expressed this exact regret. It's a kit that boxes you in and doesn't provide many options, and it doesn't perform as well as others. It's an attractive all-in-one starter kit for an attractive-looking price, but... getting started in e-cigs is an investment. Invest in quality up front and you won't be throwing money away on mistakes.
Can anyone tell me why Blu is way worse than Joye 510 or Kr8? Mostly I just want to know if the concerns I have, and my reasons for wanting the Blu, are justifiable or complete bunk.
The choice is yours, but I think you'll end up happier with a Joye or KR8. I lean toward the KR8 myself, but I'm prejudiced because I've had such a good experience with mine. Others have enjoyed their Joyes just as much. I think either is a better choice than Blu.
