I have a Boss GT-6B and I use it a lot. The chorusses and delays are good. Amp simulations CAN be good if you set them up in the right way. Quite a good octaver too. Distortions can be useful, but I prefer a separate Boss ODB-3 unit for my sound. Same with the compressors. Quite good, but a real rackmounted compressor is more flexible.
All the fancy stuff like the harmonizer and the auto slap are a big waste. Useless crap in my opinion. I also cannot get used to the bass synthesizer, but that may be because I don't do bootsy style funk. I actually did get some good sounds with the phaser on. Same for the flanger here.
So if you really need effects, want them all in one unit, are willing to spend some time setting it up and can get it under 400 euro's, this is a treat. Usability is a lot better than the Behringer V-Amp for example (which has a horribly long delay when changing effects and needs a separate pedal). The best bass multi effect on the market so far, I'd say. If you've more money to spend, compare it to the Line 6 Bass POD XT (I did not).
And ehm...don't wait for the GT-8B or a new Line 6 Product (Bass POD XT Live or something). Won't happen soon.