I have white lightning which is the same family and they've been good to me. These old white lightning 10,000 and 5,000 are more powerful and quicker recharging than an attached flash, and are wimps compared to the alien bees and related newer more powerful monolights. The whole guide number thing sorta gets thrown out with the bath water because it's intended to be an easy way to simply flash power calculations and it doesn't apply when you are using things that redistribute, rediffuse, block, and modify the light in as many ways as there are options for studio gear. Look how many umbrellas there are just for one; some are fully silvered, partly silvered, different sizes, transparent umbrellas, etc...
You'll have lots of fun with the quicker recharge times; the modeling lights on mine dim slightly while recharging so I know when it's ready and it's about a second for full power. The modeling lamp is super nice (and a dslr is good to verify the lighting proportions)
More power is good for a few reasons, and they don't apply to everyone.
LF; if you want the depth of field for a multi-person photo, you might be shooting at f22 or 32 in 8x10 or 4x5. That needs lots o light.
Group photos; you have to light a big area.
Outdoors group photos; (like a large wedding party) You need something powerful to take out shadows sometimes if you can't put a group where the lighting is ideal. You can't get too close with your gear with a superwide lens because it'll make people on the ends look fat. So you have to back up and shoot with nearly the power of the sun. Passive reflectors are blinding.
Fast recharge times. Theoretically, a b800 may recharge and be ready faster than a b400 since it is basically operating at half power. Spec sheets online would provide the details.
Lighting sports venues is also a popular use for monolights. A set of 4 top of the line alienbees will nicely light a hockey rink or basketball court.