I love the form and function of the Blu. It was my first e-cig, and during the first week, I found this forum, learned about other e-cigs and bought a 510.
I learned how to mod my 510 carts with blue foam and highping's mod to really fill the cart, and I like how the 510 tastes, but...
I picked up my Blu, cleaned out the used carts, and filled them with some tobacco-flavored juice, using some blue foam and highping's mod. I'm able to get about 12 drops in each little cup, and I cover them with those little silicone caps from the 510 carts. I use my Blu strictly for the tobacco flavor, and I find my Blu tends to have an overpowering throat hit - perfect for my marlboro fix! While the batteries die fast, the charging pack is convenient enough. The fact that it'll hold a lot of stuff (unlike the lousy PCC pack for the 510 which only holds ONE battery, what the hey) makes carrying the Blu pack amazingly convenient!
I use my 510 at home mostly, where I enjoy all kinds of flavors. When I'm out and about, I grab my Blu pack.
I have to say, the folks who designed Blu have their act together. With the pre-packed carts, decent pricing on all the parts, it's probably the most accessible e-cig that I would recommend to a new user - provided they learn how to fill their own carts so they can overcome the weak-tasting prefilled carts.
The only thing I'd like them to improve on would be... nothing! I'm happy with the Blu