Shapechange is a 9th level spell that seems to be next to useless. You can't turn into a melee creature due to the spell being concentration, causing you to drop the form. Spells/abilities can't be used as it would drop the concentration. So basically you drop your one 9th level spell to look like something cool for a few turns until you lose concentration. Am I missing something or is it really this useless?
Yes, Shapechange is very useful.
If you meant dropping concentration in melee as dropping it due to getting hit, then you should pick your forms more tactically. Change into something with resistance or immunity to the damage the foe is dealing.
The foes switched tactics and hit you with something else? Switch up and take another form with different resistances/immunities - without dropping the spell!
Got really spooked or really need to get around the battlefield more easily? Switch up into something that has the required movement speed or teleportation abilities.
And take note that, if your form allows you to make the required verbal/somatic components, you can cast non-Concentration spells without dropping Shapechange, as normal with all Concentration-based spells.
The power of Shapechange only goes as far as the caster's (and the player's) knowledge of creatures to transform into - and creativity. ;)
I've just noticed the "Druid" tag on this question. If Shapechange is being analyzed in the context of the Druid's other options, such as the amazing Wild Shape, then still, yes, Shapechange is very useful. It grants you more than just Beasts (and the specific Elementals, for Moon Druids), at the expense of being a 1-hour, Concentration spell. ;)
You need to remember, not all spells need to be justified with their power in combat, as well. This spell is extremely versatile for many out-of-combat purposes, if you know a creature that can do a thing that you cannot -- well, now you can also do that thing.
In combat, the spell still has many uses. You gain the new creature's saves, so if you pick a monster with a good constitution save, you are more able to make that constitution save. If it has a decent armor class, you might not even be hit in the first place. Finally, you can always pick something that will be out of the way, either flying or burrowing or any other way you have of making yourself harder to hit.
Also remember, your concentration spells are typically buffs on yourself or conditions you impose on the enemy or the battlefield. With this one spell, you've most likely absorbed at least one hit. (Because even if you fail that first concentration roll, the damage was still applied before you rolled it.) You've possibly made yourself harder to see, or harder to hit, or any number of things.
Finally, you're usually not alone: If you're in a party, the wizard who shapechanged into a medium sized dragon is usually going to be hit less than the level 18 barbarian that charged the enemy, or rogue next to him, or cleric, or ... You're in a group.