Seems a bit too expensive, even for Lannister. It would be worth it if he killed a character for sure but the card count dependency makes it a much less usable card. It would also be nice if you could choose instead of your opponent. Right now this just seems to be a dead card in my hand more often than not.  

Back when only core set cards were available, I used to trigger these guys quite a lot. Part of the reason was that I was always running Lannisport x3, Pycelle x2 and Counting Coppers. I think they can still be decent in Lannister mono-faction aggro with lots of draw. Even if you don't trigger their ability, they are still cheap claim soak and can be reduced with Fealty.

I was so excited about using these guys on a First Snow turn to hit a big character.

But yeah, that's the one time I'm pretty much guaranteed to have fewer cards in hand than my opponent. 

Has anyone put these to good effect? Admittedly I've only experimented, but I can't seem to meet the conditions to trigger them.

I used him some times. With Cersei/Casterly Rock you should meet the conditions in almoust every game. If you then add Tyrion you should also have the money to afford him.

Of course he rarely kills big guys, but it's a nice an additional military claim and you add a claim soak for yourself.

If you add two claim plots/cersai and trial by combat it can start to add up + running marched to get rid of the big dudes who stay. Also nice with first snow as you get played assassins back, You hit 4+ gold characters with them and have a chud on the board for a marched next turn.

I think he's only really useful if you're running Casterly Rock and Tyrion.  Otherwise he's very expensive for a non-targeted kill.  Even with CR and Tyrion out, a lot of things have to line-up in order for him to have an impact.  Usually a "win-more" card in my experience.

Though to be fair.... when are you not running Tyrion???