Seems like it would be unsporting for a DM to use this on the martial characters. 

Picture it: Your party walks into the final chamber of the demon's lair, when suddenly, your barbarian and fighter are surrounded on all sides by a giant cube of some kind, leaving your wizard, your warlock, and your druid to deal with Orcus and his Death Knights.

No...

It's unsporting to subtle this wth a bard-sorc BBEG so the counterspellers can't stop it...

It sounds like that may have happened to you.....

No, Viper. I'm the evil I want to see in the world. 

More bad design.  Dimension Door or teleportation spells and effects should simply work, no save required.  

Could this be combined with sickening radiance for a warlock?

Seems to me like a certain death for a character in 7 rounds...

How can a Warlock get the Forcecage spell? I only see spell slots up to 5th level and haven't noticed any invocations or patrons that give it yet...

Oh, Mystic Arcanum... Nice! ;D

Quote from SergeTroy >>

More bad design.  Dimension Door or teleportation spells and effects should simply work, no save required.  

Not really unless you're talking about bad design of how underpowered this spell is compared to its 5th Level counterpart, Wall of Force due to it only needing a 5th Level Spell Slot (Which has nice mileage due to the required resources to cast Wall of Force) compared to Forcecage's 7th Level Spell Slot (Which under performs with bad mileage of having to waste a 7th Level Spell Slot). In fact, Forcecage is one of the top 10 under performing spells with bad mileage just like Mordenkainen's Sword.

Quote from Vinton >>

Quote from SergeTroy >>

More bad design.  Dimension Door or teleportation spells and effects should simply work, no save required.  

 

Not really unless you're talking about bad design of how underpowered this spell is compared to its 5th Level counterpart, Wall of Force due to it only needing a 5th Level Spell Slot (Which has nice mileage due to the required resources to cast Wall of Force) compared to Forcecage's 7th Level Spell Slot (Which under performs with bad mileage of having to waste a 7th Level Spell Slot). In fact, Forcecage is one of the top 10 under performing spells with bad mileage just like Mordenkainen's Sword.

 Wall of Force requires concentration.  Forcecage does not.  This difference is huge.

Can an Echo Knight pop out of a forcecage using their shadow-thing?

 Can the caster dispel this before the hour is up?

Quote from ZachNoorman >>

 Can the caster dispel this before the hour is up?

Technically? I see no way how. Dispel Magic explicitly doesn't work and since it is not concentration you cannot stop it once it is running.

However, I think a lot of DMs will agree to just let the Caster do it if it's not having any other effect on the adventure.
If the party has the time and can just wait 1 hour until the spell is over, that would not have any mechanical difference. Might as well make the spellcasters look cool.

Edit: Then there's Antimagic Field, which is a whole 'nother can of worms. With that it should be possible, but I'm not aware of anything less powerful than an Antimagic Field or the DM's Fiat to break a Forcecage.
Edit2: It seems like Disintegration might also get rid of it. Disintegration mentions constructs made out of force like the Wall of Force, but neither Disintegration nor Forcecage mention them interacting with each other unlike Wall of Force and Disintegration clearly mention each other.

Quote from Vinton >>

Quote from SergeTroy >>

More bad design.  Dimension Door or teleportation spells and effects should simply work, no save required.  

 

Not really unless you're talking about bad design of how underpowered this spell is compared to its 5th Level counterpart, Wall of Force due to it only needing a 5th Level Spell Slot (Which has nice mileage due to the required resources to cast Wall of Force) compared to Forcecage's 7th Level Spell Slot (Which under performs with bad mileage of having to waste a 7th Level Spell Slot). In fact, Forcecage is one of the top 10 under performing spells with bad mileage just like Mordenkainen's Sword.

Wall of force gives creatures inside total cover from any effect originating outside. A Forcecage in the shape of a cage does not, allowing you to bombard the helpless target with fireballs, or kill them slowly with sickening radiance as Forcecage is not concentration.

My monk is currently sitting in one now. During the final battle.

You are talking about the infamous combo I like to call:  "Will it Blend?"

Sickening Radiance is a nice option because if the enemies fail every save, they're dead in 7 turns (6 if you can get a Bard or Wizard to cast Forcecage for you).  Another good options is Maddening Darkness (essentially blinding opponents).  It's basically certain death for non-spellcasters.  Taking guaranteed damage from the AoE (plus the attack spells you're using) means that nothing is likely to survive that.

Correct me if I'm wrong, but couldn't shapeshifting magic like Polymorph get someone out without a Charisma save? A druid could just Wild Shape into a fly and exit through the bars, since this isn't interplanar travel, I don't think a save would be necessary.

That only works for the cage shape though...

But you are right, the sickening radiance + forcecage combo does not work on shapeshifters

Someone could throw a moonbeam in there to ruin any shapeshifters dream.

Edit: Also as a GM I wouldnt allow for a 1HP wildform or polimorph while inside a continuous damage effect but that would have to be a discussion itself.

People are saying underpowered and stuff. However, as a Warlock, here's what you can do with this against non-magical enemies.

1) Forcecage the enemy.

2) Cast Hunger of Hadar under the enemy.

3) Sit back and eat lunch while the enemy is blinded and surrounded by soft whispers and slurping noises whilst taking 2d6 cold and 2d6 acid damage for 10 rounds. If he isn't dead by the time Hunger of Hadar finishes, cast another one :-)

*evil laugh*