GoatBoySteve: I disagree that dualing isn't worth it before level 9, some characters I dual immediately. All you really gain at lvl 9 is an extra point of weapon proficiency and a handful of hit points. The longer you wait to dual the longer that character is basically useless. You get an extra 1/2 an attack at 13, but that character will be mostly useless for a good chunk of the game.
Yes, you can in fact have fun if you dualclass right after reaching fighter level 2, those 2 levels are enough to double physical damage output compared to pure mage or thief.
But it's definitely worth waiting until fighter level 3 for weapon mastery and bonus HP since the extra XP costs are minimal (less than 1 percent of the XP required to get an additional level of mage or thief after reaching level 11).
The next big fighter boost is at level 7 when you gain an extra half attack per round, you can complete dualclassing during BG1 but you won't be able to backstab for quadruple damage or cast level 5 mage spells during BG1. The dualing period takes a very long time.
Waiting until level 9 in BG2 only adds HP and an extra proficiency point, but the dualing period will be much shorter, in BG2 you'll get the XP required for mage or thief level 10 much faster than the XP required for mage or thief level 8 in BG1.
You could dual right at the start of BG2 but since you'll probably start with 161,000 XP you only need 89,000 XP for the 9th level.
Waiting longer in fact isn't worth it in a normal game. Fighters get less HP than mages or thieves for reaching level 10 and the next boost (the extra half attack at level 13) will cost 1,000,000 extra XP.
If you dual to mage at level 13 you'll be a pure fighter for 40 percent of the game and a low level pure mage for 30 percent of the game.
And when dualing is complete you'll still have 1,250,000 less mage XP than a pure mage so you'll have less spells per day at a lower casting level and also get HLAs much later. The inferior casting ability will drive you towards cheating when you realize you'll never reach the 8,000,000 XP needed for this build to max them.
When dualing to thief consider that 1,000,000 XP are 4-5 thief levels including HLAs.
I'm a massive fan of Kensai. The lack of armor scared me off initially, but they don't need it. They deal insane damage and with solid hit points and a Shield Amulet they are plenty sturdy. In some ways the armor restrictions are cool, since it forces you to use items you might not normally use.
When playing a kensai I found myself hiding behind another party member to reduce the need for healing.
The shield amulet costs money for recharging so each time you use it you loose money (I'd rather have the party druid cast barkskin). And even in combination with the +2 kensai AC bonus it's still not as good as non-magical full plate.
For leading the party into battle from the front berserkers are the better choice. With full plate they're hit less often and their enrage ability grants lots of bonuses and immunities which help a lot especially in the harder battles.
The kensai advantage is that their damage bonus keeps improving as they level up.
Magic does become more important in BG2, but you could stay a single class fighter and just recruit a wizard or two into your party. If you're soloing I'd definitely go Kensai/mage or Berserker/mage if you are of a Chaotic alignment.
There's in fact no need to dual to mage just because it's teh most powerful choice for a solo character. If you have an abundance of mages but are lacking fighter or thief options dualing to mage is a selfish choice which improves the character at the expense of the party and probably not the most enjoyable way to play the game.
