I tried to build the Garchomp-Talonflame deck and played it online for the best part of a day- I made a post about it here:

https://www.reddit.com/r/pkmntcg/comments/55z0aa/good_new_budget_deck_garchomp_talonflame/

Although it's just based on online play, I'd say:

The first thing is that it is a bit inconsistent, and a bit slow to get going. Games where you don't start Talonflame can be tough, especially when you go second. It's probably less consistent and slower than every top deck except Greninja, and so even against decks where you have a favourable matchup, you still lose a little more than I would like.

That being said, it can be great against any deck that uses mainly EX attackers: Mega Mewtwo, Mega Scizor, Mega Gardevoir, Darkrai-Giratina... if you set reasonably well against any of those you have a very good chance. In decks with a mixture of EX and non-EX attackers like dark decks and Volcanion it's much tougher. Likewise a deck that uses non-EX attackers with EX support Pokemon (Rainbow Road). You basically have to hold on and hope that you can Lysandre up the EX targets to take your prizes before they can KO 5 or 6 Garchomps. Against Greninja, your only hope is that they get a horrible start and you get a good one- if they get going fully, forget about it.

Overall the spreaad of matchups is okay but not great: favourable against 4 or so very popular decks, unfavourable but not an auto-loss against 3, terrible against 1. Arguably the better decks have fewer bad or terrible matchups, but that's what you get with a budget/rogue deck. But you also have to compensate for the fact that it's a bit slow and inconsistent too.

So I guess... it depends where you are coming from. I can't see it being "the play" in many tournaments because it's pretty risky hoping to run into Volcanion, Greninja, Yveltal or Rainbow Road just one or two times on the way to top cut. It's possible, I guess: you could have a run of six games that were all Mega Mewtwo, Gardevoir, Darkrai and Scizor... but not that likely I think. So if you have the resources and are trying to pick the very best deck, I don't think you'd pick this. But, if you wanted to play a rogue deck, or didn't have the resources for a more expensive deck- sure. There are not many other decks, if any, that give you that kind of spread of matchups for so little money.