1. Is it a reliable weapon?
If you use charge attack, inflicting status is very reliable. Normal attack, not so much.

2. Does the shock inflicted on enemies spread to you and your allies?
Yes. It isn't usually a problem if you use charge attack as it has some knockback.

3. Would you recommend it for all-around use(except for beasts and gremlins of course)?
Shock is a good status and generally safe if you can knockback your enemies and not into your buddies. If it's your only elemental sword of course. But compared to the Divine Avenger and Glacius and Combuster... which are way easier to get and upgrade... are you willing to spend hours in Blast Network (and maybe Krogmo boosters) for a cool novelty item?

I use the Boltbrand and would strongly recommend it. It is my first four star weapon so I don't have too much to compare it to, but I find it does well against almost all units and the charge attack inflicts shock on all enemies nearby. I often use the charge attack since it can often one hit kill enemies, or if the strike of the charge attack doesn't the secondary explosion sometimes will finish of a unit. I haven't noticed the shock spreading to me or my allies, but my method tends to be charge, attack and immediately kill or in arenas if there are a lot of units chasing me, charge, attack, run, charge, attack, run, repeat.

As a secondary weapon you might want to consider a flourish. It does really well against beasts, and fiends (although the boltbrand isn't too bad againts them). Gremlins you probably won't encounter too often anyways. The first attack of the flourish covers a larger area than the boltbrand and the charge attack with a little practice can be really effective (think it does double normal damage, but it often puts you in the middle of an enemy group). I love lining up a large group of fiends behind a closed door waiting for someone else to open it then taking almost all of them out.

I posted the following in another thread, but I think it applies here as well. Some advice I read on the forums was trying out the 3 star versions of the swords to compare. I did that and found the iceburst brandish very frustrating to use in a group. Soloing I am sure it would be very useful, but most damage is done when a unit unfreezes and most of the time your allies will break the freeze almost as soon as it happens making the bonus of a charge attack pretty useless. This may get better as you get into later tiers and users are more knowledgable. Fireburst brandish I didn't give as much time to try out, but for some reason the status effect didn't seem to happen as reliably to nearby units as with the shockbust brandish and I was thinking down the road to FSC. I like the Boltbrand because it is sort of a hybrid of the two other swords not quite as good at damage over time or impairing movement, but does both fairly well, the spreading shock is also pretty cool. I have also read if you attack an enemy that is receiving a residual shock from your attack they receive greater damage, so maybe there is a little bit of the status of poison mixed in there too.

I try to use the charge attack as much as possible, especially in an arena with a large group chasing you and the secondary explosion I have shocked most of the group without getting hit (although I am sure the same could be said of the other options).

Another consideration is whether you want to put in the time (and money lost) playing Blast Network to get the coins.

lol nobody would buy a boltbrand even if it did have a UV
great job necroing a thread for your shameless advertising

...that post is 3 months old. That price is also bad, although you are welcome to try and find someone who would haggle with you.