I've played Tom's [color=#282828][font=helvetica, arial, sans-serif]BSX Allegro 5 but only briefly so I can't really compare it to the Yamaha, I've not had the opportunity to play a 4 string.[/font][/color]
I'm on on my second SLB200, I think they come close to the feel of an upright acoustic but the sound is always a compromise.
However for portability and zero feedback they score highly.
There's loads over on talkbass if you've got time to read it all.
Check out Max Murray, he gets a convincing upright acoustic sound out of his SLB200, solo about 2 minutes in.
[media]http://youtu.be/vm564Px7Ino[/media]