While people are on the subject of what's worthwhile having...

Are there any feats that you think are real 'must haves'?

I never really pay much attention to them but i've noticed that quite a few don't make a difference

There certainly are some good feats and some terrible feats. Must haves depend on what sort of character you have.

Weapon proficiencies are pretty important (vital) for fighter characters but next to useless for spellcasters. Power Attack, Cleave, and Improved Critical are also high on the list for your warriors.

The various spell feats are important for casters. Spell Focus: Evocation is a favourite for arcane casters with all the firey damagey spells they use.

Forester is useless for everybody and Improved Initiative doesn't work.

You definitely should pay attention to them because they can make or break your characters at higher levels.

For frontline melee charecters pick power attack, cleave, dirty fighting, improved critical, heroic inspiration.
The elemental feats are almost necessary and spell focus in a couple of schools too.
Either your decoy or your HEALer could get dash. Blind fight, dodge, expertise are good for your decoy.
Armoured arcana and combat casting for spellcasters.
Lingering song for bards, extra rage for barbarians, evenom weapon and hamstring for rouges, and shape change shambler for druids are quite useful.

Dont bother with shield, armor and weapon proficiencies except those which your classes give you. Exotic weapon:bastard sword may be useful for 1 charecter to pick.
Forester is pretty useless. Snake Blood and toughness, even more so.
The various skill and save improving feats are pretty useless for anyone other than a specialised decoy, then also only if you find yourself being hammered continuously.

Deflect arrows is only useful if you use a single handed weapon with no sheild. Mercantile background has hardly any use.

I like the concept of feats. Along with skills, it helps you to create a more customised character.

I've finished the game loads of times and never paid attention to them. maybe i should start...

You'll get more enjoyment out of the game by making different characters with specific feats. It also makes it easier at higher levels of difficulty.

Casters*: Improved Spell Focus: Evocation, Improved Spell Penetration, Spirit of Fire.

This applies to wizards, sorcerers, druids, morninglords of Lathander and other priests connected with fire via domains. For priests connected with lightning (Talos), Scion of Storms instead. Or you can combine more than one element especially in sorcerers (one of mine had both Spirit of Flame and Scion of Storms). Everyone with magic that aims at enemy targets needs Improved Spell Penetration. Other interesting Focus for spells might be Transmutation. Take Subvocal Casting if you have spare feats.

Tanks: Power Attack, Cleave, Greater Cleave, Improved Critical Attacks, Weapon Focus, Weapon Specialisation. Fighters also Blind Fighting.

Paladins: Fiendslayer

Any archer (crossbowman doesn't count): Rapid Shot, Point Blank Shot

Scouts, soloers, spec-ops mages etc: Dash

Dexterity-based fighters: Weapon Finesse (of course), Weapon Focus, Weapon Specialisation (need to get the damage somewhere), Power Attack (need to overcome resistances and sometimes this is difficult with small weapons), Improved Criticals. Like also archery feats. BTW, I really suggest starting a DEX-based fighter, ranger or barbarian as a rogue and taking two levels if you can get away with it without multiclassing XP penalty. Even wizards will benefit from two rogue levels.

Fighter/wizards, fighter/sorcerers: Weapon Focus, Weapon Specialisation and Improved Criticals for Mordenkainen's Sword (7th level spell)

Clerics: make as fighters or as casters, just don't mix unless you really know what you're doing. I had a great morninglord of Lathander without a single combat feat and he still fared well in combat (after all, priests get lots of spells to improve combat capacity)

Druids: make as DEX-based fighters or as casters (fire-based)

Bards: Lingering Song, Improved Spell Penetration, focuses on spell schools you are using, most likely Transmutation, elemental feats and focus on Evocation if you are using such spells but bards normally don't.

For wizards/sorcerers:
Spell Focus/Greater Spell Focus: Evocation -- take these early as your most powerful spells in the early to mid levels are Evocation spells.

SF/GSF: Necromancy -- take these a little later as at higher levels your most powerful spells are Necromancy spells: Horrid Wilting and Wail of the Banshee.

Take all four of the elemental boosters. Spirit of Flame first, followed by Aegis of Rime, Scion of Storms and Aqua Mortis.

You may be able to get some use out of SF/GSF: Transmutation. It works well with Slow and Disintigrate.

I do not like charm type spells so do not take SF/GSF: Enchantment. The only exception would be with a high wisdom Dreadmaster of Bane build. Such a cleric would have DC's high enough to make Enchantment spells work most of the time.

Unlike Chevalier, I would not take the Spell Penetration feats. Most monsters in IWD2 have either no resistance or very high resistance. If you are battling one of the few high resistance monsters, simply use Lower Resistance or use one of the spells that ignore magic resistance.

For my part, since my strategy involves using a magic resistance possessing decoy who is bombarded by the rest of the party, Spell Penetration would actually be worse for me than for the monsters.

Finally, if the caster in question is my main bombardier I will usually take Subvocal Casting to insure that he never gets Silenced. I have noticed that the priests in the Ice Temple love to throw Silences at my casters.

Hope that helps some.