Based on the classes possessed by the PCs in the question, it seems like the campaign's PCs aren't all tier 1 and 2 or heavily optimized to compete with tier 1 and 2 foes. That's totally okay, and, in the interest of full disclosure, neither are PCs in my two current campaigns. I preface this answer this way because I absolutely don't want readers to think that the headlines below are implying that a high-powered campaign is somehow better than a lower-powered campaign! I totally respect your playstyle, and it mirrors my own.
In a higher-powered campaign, LA is only very rarely worthwhile…
In the abstract, level adjustment is pretty close to the worst thing that a player can do to a PC. It takes a lot for what are, essentially, monster levels to equal character levels. My tendency is to evaluate any level adjustment by asking If this batch of monster abilities were available as a 1-level dip, would my PC consider taking it?
For the pixie, this becomes…
- If taking 1 level of pixie granted only –4 Strength, +8 Dexterity, +6 Intelligence, +4 Wisdom, and +6 Charisma but no base attack bonus, saving throw bonuses, skill points, or hit points, would I consider taking it? Maybe. +8 Dex is a lot of Dex, increasing AC to a point where it makes up for a lack of hp, and the +6 Int makes up for the loss of skill points. The increased Dex and Wis makes up neatly for the lack of saving throw advancement. This isn't terrible.
- If taking 1 level of pixie granted only an at-will supernatural ability like the spell greater invisibility but nothing else, would I consider taking it? Yes. Even though the spell greater invisibility, unlike its lower-level lesser counterpart, can't be used on objects, the ability is an actual, for-reals, full-on supernatural ability, making it immune to dispel magic effects and not provoking attacks of opportunity. It's duration is the character's Hit Dice in rounds, making it so at high levels the ability becomes a pre-combat buff spell for the whole party as combat's only gonna last a few rounds anyway.
- If taking 1 level of pixie granted only the pixie's spell-like abilities and fly speed of 60 ft. (good) and nothing else, would I consider taking it? Probably? Its spell-like abilities are all 1/day and possess a fixed caster level of 8th, and the Monster Manual errata removes from the pixie the spell-like ability polymorph (self only)… which could've made this a Yes. This makes the only interesting spell-like ability permanent image except that there's no olfactory or thermal elements because that's why. The fly speed is an escape-and-evasion ability, but it's a necessity for everyone (especially at upper levels) so this is merely saving the PC up to 30,000 gp. However, there are a lot of spell-like abilities here, and having them—even once a day—means not needing to get them another way.
- If taking 1 level of pixie granted only the pixie's racial skill bonuses, the feat Dodge as a bonus feat, a +1 natural armor bonus, low-light vision, DR 10/cold iron, and SR 15 + class levels, and nothing else, would I consider taking it? Barely. Most of these are gravy, but, while DR 10/cold iron or SR is usually a class's kind of bad capstone class feature, for this character—with its lost saving throws and hp—, those are both decent. That SR is high compared to most creatures. (Creatures typically have SR equal to their CRs +10, but the pixie PC's SR is equal to its effective CR +11 and remains so as the pixie advances in levels; a drow PC, for example, has SR equal to its effective CR +9.) However, SR comes with its own problems, and it's complicated. DR 10/cold iron will mitigate attacks by weak monsters, making up some for the loss of hp… but it won't save the creature from, for example, uberchargers or mailmen; of course, very little does, and that's kind of the point.
While there's no way this player would try to play a level 1 pixie druid PC in any group of 5 level 5 PCs, playing a level 10 pixie druid in group of level 14 PCs seems like it might be a reasonable life choice… if the other PCs are approximately equally optimized. And they are.
See, a typical druid 14 in a group of level 14 PCs contributes significantly to that group of PCs, not only because that druid casts 7th-level spells (heal, greater scrying, master earth, slime wave) but also because of its animal companion and the supernatural ability wild shape.
That is, in a group that includes an
artificer 1/rogue 10/warblade 1/warlock 2, a barbarian 2/fighter 2/ranger 2/Rokugan ninja 2/warblade 2/people's champion 4, a cleric 14, and a wizard 14, a level 14 druid fits right in. But in the same group a level 10 druid that has higher-than-normal ability scores and the big pile of random special abilities from being a pixie will probably leave the pixie druid 10 feeling useless most of the time, hovering invisibly above the fray, sniping with her weak spells, hoping the monsters will fail saves so the player can say he's contributing.
…But in your campaign, it seems like a pixie druid 10 would be fine
In the group you've assembled, a straight-up level 14 druid—if played to the hilt—would be overpowered. That druid's wild shape would make that druid better at roguing than the rogues, its spells would make it better at casting than the casters, and its animal companion would make it better at fighting than the fighter. Taking the level adjustment hit that comes from pixie puts the PC in the same range as the other PCs and makes it so the druid's out of combat utility won't totally overshadow the other PCs' abilities… although at-will supernatural greater invisibility will come close, the pixie's player should be encouraged to consider using it out of combat on others first: because they're actually higher level, those other folks' skill modifiers will be higher!
In sum, yes, in a typical campaign, level adjustment is almost always to be avoided at all costs. However, based on the other PCs in your campaign and the levels at which you're already playing, suffering the level adjustment in exchange for playing a quirky PC that can still contribute seems like a reasonable compromise that would be more fun than just adding 3 or 4 levels of rogue or fighter to 10 or 11 levels of druid and calling the PC done.
Note: In two campaigns I'm running, a PC can't take twice in a row a class level in the same tier 1 or 2 class or in another class that advances a tier 1 or 2 class's casting or manifesting. PCs are level 13 in both campaigns. In other words, the PCs in your campaign look sort of like mine if you turn your head and squint. (Mine multiclass more. A lot more.) That makes my experience here practical not just theoretical. This DM would worry about the pixie being 4 levels behind on hit points with a stat array that may see a Constitution in the lower teens—monsters can hit hard at this level and being 25 or so hp behind the curve is a big deal when facing hill giants with class levels—, but the pixie's enormous Intelligence score and the Dragon #319 feat Faerie Mysteries Initiate (58-9) should solve that if another PC can be convinced to take the feat or if the DM allows the pixie a cohort that possesses the feat.
