If you're already winning, Jace usually pushes you far enough ahead to seal the deal. Its one of the best possible topdecks when both players are low on resources
If you are in a losing position, it is not as impressive but can occasionally help pull you out
Jace changes how lands will be played. It may be correct to hold lands in hand and wait on that fetchland activation in the chance you draw Jace, use brainstorm, and then shuffle the extra lands away.
I'm trying him in Ux Midrange/Control builds (UWR, UR, Foretold End, Grixis and RUG some day, maybe even Scapeshift some day) and one of my older midrange brews, Bring to Light Madcap Emperion. (Thank goodness I could finally rip out the low-impact See Beyond from that deck.)
JTMS seems to improve my match-ups, produce good CA, and get run over or force me to throw meat shields in his way to protect him. He doesn't tend to win the game by himself, though.
He generally gets run over by Bloodbraid Elf decks. I haven't tried Jund or RG Titanshift with BBE against him yet, but he hated RG Ponza and Copycat with BBE.
Gx Tron also seems to have a field day with JTMS, as they have 8 or more ways to get rid of him immediately, and they often go over the top of his CA.
Interested in modern format experiences with the card. What deck are you trying him in? Has Jace made common matchups better? Worse?
I've faced him in Eternal Command, RUG, and Grixis. Honestly the RUG build simply stomped me.
Spirits
If you are in a losing position, it is not as impressive but can occasionally help pull you out
Jace changes how lands will be played. It may be correct to hold lands in hand and wait on that fetchland activation in the chance you draw Jace, use brainstorm, and then shuffle the extra lands away.
JTMS seems to improve my match-ups, produce good CA, and get run over or force me to throw meat shields in his way to protect him. He doesn't tend to win the game by himself, though.
He generally gets run over by Bloodbraid Elf decks. I haven't tried Jund or RG Titanshift with BBE against him yet, but he hated RG Ponza and Copycat with BBE.
Gx Tron also seems to have a field day with JTMS, as they have 8 or more ways to get rid of him immediately, and they often go over the top of his CA.
Spirits