I'm guessing this works as a 'double Mobile' when triggered off another Mobile unit? So trigger the first mobile unit's move, trigger Vyper's reaction to follow across, then trigger the Vyper's mobile to skip to the next planet along? Has potential.

Mobile all happens at once, so this unit can't mobile after it's reaction.

No, but you can do it the other way around. Use the Vyper's Mobile to move it to a planet another Mobile unit just left, then trigger the reaction.

While the new Vyper is great for Baharroth, it can turn out to be a double edged sword for him..

A fun way to use this unit is to chase down opponents' mobile- or moving units if they try to get away from it, and seeing as there is no limit to the reaction there really is no escape for anyone.

That goes for Baharroth facing one of these as well..

It is, no doubt about that. The Bonesinger Choir is first and foremost what makes the Vyper viable and the Choir is potentially useful in any setup that can run Eldar vehicles

(Sae'lum Enclave is another approach; you'll need a Tau warlord for that though.)

I run the Enclave + Bonesinger in my Shadowsun deck. That way I'm guaranteed to see at least one of them in all but the most exceptional opening hands

How many Eldar vehicles/drones do you run, Kaloo? The Choir only starts to pay off when you put the second unit into play with it. Do you have enough to make that worth the cost?

The Choir only starts to pay off when you put the second unit into play with it. Do you have enough to make that worth the cost?

I think that all depends on how you look at the idea of something 'paying off' FightingWalloon 

Ressource-wise, that is from a strictly mathematical point of view, then it's certain that a dividend comes when you deploy the second unit using the Choir and onwards. But pay off, or value, can be derived from other things as well and I believe it to be important that we also remember to consider other kinds of gain than the mere mathematically invest/return-ratio in our consideration of cards.

For instance, I'd argue that the strategic pay off comes sooner than the deployment of the second unit. Should it appear in your starting hand alongside an Eldar vehicle, the strategic advantage of deploying it and thus - assuming it isn't destroyed - possessing the option to deploy a heavy vehicle unit, like the Vyper, at the start of any new round gives you a pay off that might outstrip the value of the monetary pay off in my opinion.

Furthermore, even though all factions potentially have access to these, there are currently only about five cards in the game that can remove or destroy (counter) the Choir, so odds might be decent for it to survive either a few rounds or entirely as a thorn in the opponent's side. The lack of counters for it might represent value as well.

(Though it is likely that these cards are on the rise of course).

Lastly, whether the Choir is 'worth the cost' or not depends just as much on the unit you deploy using it. Here the Wildrider Vyper is a very good example I think: expensive, for sure, but with a solid body and a nice keyword plus a formidable ability indeed. If you deploy one of these turn one to planet two then you already threaten three planets. Next turn comes the Wildrider Squadron (or vice versa) and then you've got an excellent deterrent, potentially turning commitment into a nightmare for your opponent

How many Eldar vehicles/drones do you run, Kaloo? The Choir only starts to pay off when you put the second unit into play with it. Do you have enough to make that worth the cost?

I think Skyknight and I are on a similar page. The way I see it, even if you only use it once you get an immediate deploy stall, which makes the placement of said unit easier. I run quite a few targets with a particular focus on mobile/moveable units since they all help the Vypers (as well as Shadowsun's event). Two notable includes are the WraithKnights and the Soaring Falcons. The latter is a 4hp improvement on the Trailblazers for 1 less cost (both of which are great Drone Defense System targets), whilst the former helps to exhaust everything on the planet adjacent to the DDS unit, allowing them to mobile in and deal out the hurt (even if there's no battle). For The Tau'va helps this even more.

Regarding how much I use the enclave/choir, in the few games I've tried out I've managed to get a use out of 1 support every turn, with 2+ supports being used on the turns that they're available. The only time I haven't had consistent use out of the supports was the game in which I didn't draw any (first ECT threw 2 of each ....)
 

As a non-Eldar player I can tell you that this is one of the most feared cards the Eldar possess! Oh, how I hate to see this unit in play! It can terrorize command and your Warlord and basically ruins your day.

5/5 card.