Ok, for the quick summery, read this part. If you want more in depth, read past this. I've played Baldur's Gate since 2003 or so finished the game twice total. Only five years after it originally released in 1998. This game is basically my childhood. Over all, this game is much like the original with additions of a new class and the half-orc race of Baldur's Gate 2 and now, there's even more customization in the stats. Over all, the story is the same and enhanced on with new characters. If you're into hard-core RPGs that will last you days, buy this game. It's well worth it. But if you want something that has less time investment, don't go for this.
Now for in-depth
Character Customization: Now, this is an old game, this isn't Skyrim. Your character customization won't be ranging from weight to dirt color. The customization is in the stats: Strength, Dexterity, Charisma, Willpower, Intellect, and Constitution as well as what weapon proficiency you have from one handed sword to katanas, from two handed swords to halberds and from bow to sling. Then if you chose a caster class what spells you have from the beginning. You can chose your class from the traditional: Fighter, mage, thief to things like cleric, specialized mage, multi-classing, ranger, ect. Then race (which has different stats for each) from human, dwarf, elf, half-elf, gnome, halfling, half-orc, ect. The only physical customization of the character is in the portrait, gender, hair color, skin color and clothing major and minor colors. That's it.
Gameplay: Now, the gameplay is DnD combined with RTS. You control your party of 6 max, including your character (who, if dies, is game over). You can pause whenever to cast spells, which is turn based. I.E. It could take two turns to cast a Heal Minor Wounds spell or a turn to cast a magic missiles or even five to cast a lightning storm or summoning spell. You can loot items from weapons (which CAN and WILL break by the way. So ALWAYS have a spare!) to armor, to enchanted items which you'll need a mage or the money to identify what they are (don't auto-equip these assuming they're better. They might be cursed items!) If you're a fan of DnD you'll love the gameplay.
Story: Without spoilers, the story is amazing and in depth. On disk it spanned over 5 disks and a sixth for the expansion, so the game itself, for the day, was MASSIVE. Games now are 8-12 hrs, possibly a tad more to finish the game, maybe even more than that.. If you want 100% and to actually finish this game successfully, you'll need about a week or two. So prepare for time investment.
Graphics: This game was made in 1998, don't expect beautiful sights like in Assassin's Creed or Mass Effect (I know, two different genres entirely, but best comparisons I can thing of). The graphics are decent enough and good for the time it was made, but the story makes up for the graphics. If you want eye candy, this is the WRONG game for you.
Size: The game spanned over five, even six disks if you had Tales of Sword Coast. So the game is long. The size of the world is rather decent, numerous side zones, towns and cities from where you start in Candlekeep to Baldur's Gate itself, the world is almost all open to exploration. every building with a door, every mine shaft and tent. As long as the cursor changes to a wheel for travel or a door, you can enter. Might not get a good welcome, but you can enter.
Difficulty: The game's difficulty CAN be set, but even on easy it's fairly challenging. Not much more I can say on this factor.
Over all, this game is well worth it and a great buy.