I really like this card now I've played it, although it seems divisive. Quick rules question though - if I play this and it would take me over 10 tokens on the field, do I choose which planets get a token, or does it cascade down from the first planet? So I have 6 'gants dotted around, play this when there are five planets out - who decides which planet doesn't get one?

That's... a really good question. Without thinking I've been assuming that the player controlling the card controls how the partial completion resolves. However, now I think about it, this sounds very much like five termagants entering play being a simultaneous effect, in which case the choice would go to the player with initiative.

Can you ask on the rules forum and post the answer back here for future reference?

This seems similar to the Snotling Attack situation:
 

Additional related question:
 
If you have eight Snotling tokens in play, can you play a Snotling Attack? I just note it says put four into play rather than "up to four", and as you can't do that are you allowed to play it?

You may. As long as the game state changes, an effect can be initiated, which means that you can initiate even if you're at 9 Snotlings in play already and only 1 comes into play. Once initiated, you must execute as much as you can though (RRG Card Effects section), so if you have fewer than 7 Snotlings in play, then all 4 must come into play.
 
The "up to" clause would be used if the intention of the effect was to allow you to choose between 1 and 4. You'd still have to abide by the max 10 in play rule though, in this example.

So I'd say the player playing the card is the one to resolve the partial effect.

Not the same question though: that clarifies that a partial effect occurs, but not who makes the choice. 

I'm 99% sure its correct that the player controlling the effect makes that choice, but not absolute on that.

Anyway, as I'm online and bored, I've made the query:

http://www.cardgamed...es-the-planets/

Nice one, thanks dude. Just saw the answer which is the best option - so in the specific situation this happened I had 6 other 'gants and knew I was going to lost the first planet. So I can just avoid that and have a full ten lined up for next turn.

I think this card's great, cheap, powerful effect that can turn very dangerous with the hive mind boosters. I don't quite understand why it's not a feature in every Swarmlord deck but then I've only been playing with it for a couple of sessions.

Similar reason as Snotling Attack doesn't make every deck. For sure, a bunch of tokens for 2R is a great thing, but spending resources on anything that doesn't immediately help you with command or immediately help you win first planet is always going to be slightly diminished in value. Spawn Termagants is very much a long term game plan thing.

The other thing I noticed is that its quite easy to hit termagant cap if you play this card with Swarmlord, doubly so if you use Gravid Tervigon too. While we might say "that's great!" its also a possible sign of inefficiency and a sign that the 2R1C could have been used to a goal other than something you were going to achieve anyway.