On the whole, I'd say that the wrong choice of multiclassing anything will seriously compromise any character. Druids aren't particularly any worse in this respect than any other class.
However, experience with one of my players does suggest that multiclassing two spellcasting types is a sub-optimal choice. Especially so when that choice is part divine and part arcane.
To set the scene, in the current party (8th, pushing 9th) we have the following characters:
Elf Cleric5 (god of knowledge)/Fighter3 - will take Ftr4 next level, and is looking to prestige into something like a master marksman class.
Human Rogue5/Wizard3 - reverting back to Rog for future levels, just wanting to dip and pick up a few useful spells.
Human Wizard7/Barbarian1 - reverting back to Wiz for future levels, but wants to head in the direction of demonology or necromancy. The Bbn level was taken simply to represent somewhat of a temper (and incidentally shocked the hell out of me when it happened...)
Half-elf Barbarian5/Ranger1/Fighter2 - looking to prestige into some sort of bounty hunter/vigilante type. This is the party frontline fighter. Assistance is given by the cleric (usually ranged) and by the rogue (usually melee w/ sneak attack). Party leadership is split between this character (for combat/tactical assessments) and the druid (for interacting with those outside of the party).
Finally, we have the Druid - a human Druid4/Sorceror4. Unlike most of the others, the multiclassing took place from the start of the campaign (they all started at second level) and has progressed in mostly equal parts as time has gone by. The other players all chose to progress a single class to a point where they were happy with its power before branching out. Consequently the Druid has been feeling somewhat left out, being only effectively half the current party level in either class.
To my mind the Druid is similar to the Bard - most effective in a support role. Once it was shown (through play) that a suboptimal choice of multiclassing makes for a poor single-purpose character, the druid player has fallen into a more obvious supporting role, and has consequently become considerably more useful to the group. The druid having recently been equipped with a couple of useful wands, the cleric can now focus on ranged support and not on the role of battlefield medic.
Don't get me wrong - the druid/sorceror *is* an effective character - a diverse range of talents for starters - but hasn't shown to be that useful in the middle of a fight. However, when the party has been deprived of his assistance there has been a noticeable loss. As a support character, really each of the other main four has been receiving an additional 25% boost to their effectiveness, which when it isn't there can be missed. Fights take longer, the cleric has to do more healing, there isn't the range of utility spells available, and so on.
Use of the Aid Another action has probably been the most useful thing the druid player has done in a fight - giving the front line the option of another two points of damage a round (by converting the +2 bonus to hit with Power Attack), or by playing target (the stacking of decent Dex, Mage Armour and Barkskin means the druid doesn't get hit very often), and the use of his animal companion (a wolf) in assisting the rogue with flanking and hence a sneak attack nearly every round are all helpful contributions to the rest of the group.
I guess in summary then, it's not so much that multiclassing a druid is a bad idea, but just that taking a 'secondary' or support character and broadening their range by multiclassing is obviously going to affect their effectiveness.
With hindsight, there may have been a more noticeable effect if instead of increasing druid and sorceror levels in parallel the choice had been made to advance as one first and then the other - only with this next level will the druid actually be able to wildshape, for example.
Personally I think druids rock, but then I've always been fond of them, right from way back in 1e days.
And to address a final point - that of companions and/or familiars, I made the decision to bend the rules slightly and to allow the 1/1 character to have a single animal companion that is also a familiar. Taking a wolf I granted a +2 bonus to Wilderness Lore as the bonus from having a familiar. Didn't appear to be too broken to me, especially since I only allowed this combination for campaign reasons - both campaign and character backgrounds suggested this was the most sensible route, rather than a small menagerie of critters. Note also that a greater part of the benefits of a familiar are actually those that benefit the familiar rather than the master, unless both are within 5' of each other. When the wolf goes racing off the druid loses some of the benefits, and it was stated that at the beginning of the campaign that familiars/animal companions/henchfolk/etc come under the ultimate control of the DM (me) to avoid any potential abuse.
