I generally like many of the multiclass options in 4e and one thing worthy of a mention is the flexibility it offers over a campaign rather than just in any individual situation.
Multiclass characters love to level up! Every time you level up you can swap all your multiclass powers for other powers in addition to being able to retrain, which provides all sorts of options.
For example a character with 3 power swap feats in fighter could upon leveling, after finding that the player of the recently dead cleric has decided to roll up a wizard, swap all 3 powers at once to 3 that provided a healing or regen benefit such as boundless endurance and victorious surge
The next level the party discovers they are about go on an adventure to fight undead so the same character retrains the mc feat to maybe paladin and then swaps 2 powers to radiant smite and martyrs retribution which deal radiant damage (still str based) and the other power back to their original class (i'll let you worry about the back story)
A couple of levels later the undead adventure is over and the character swaps back to fighter taking some now higher level powers
This flexibility is very hard to gain in any other way - effectively being able to change up to 4 powers per level up including retraining.
Paragon multiclassing as presented is terribly weak, losing the AP bonus that is built into all PP's, and only learning understrength powers.
I agree that losing the AP bonus and other abilities is not made up for by the swapping of the at will but I disagree that the powers are understrength. A 7th level encounter, 10th utility and 19th daily is about right for a paragon class. Most paragon classes have 1 cool, 1 ok, and 1 pretty meh power I'd say. At least with paragon multiclassing you can pick the best. Lets be honest - all powers are far from equal
To continue the fighter theme, imagine a paragon class with rain of blows, strength from pain and quicksilver stance. You would snap it up (we're all powergamers right ;-) )if it had even a half decent AP power to go with it (of course it doesn't which is a shame).
It's kind of ironic, really; all that effort to make a new multiclass system, and it ends up being just like 3E. Non-casters do fine with multiclassing, while casters suck at it.
I reckon thats just because there are 5 classes which key off strength with lots of secondary CHA and WIS effects. And only two of INT. Hopefully PHB2 will improve this
But in 4e (and GW) style multiclassing, the best I can hope for is a 50/50 hybrid.
By 10th you can get about 25% there, by 20th its close to 50% and in epic you can make it to nearly about 85%. You can create a fighter which has no fighter encounter powers, no fighter daily powers, 4 fighter utilities and 1 at will. Meanwhile it has 4 encounter and 4 daily powers from up to 3 other classes and 3 utilities from 2 classes. The main problem is having to wait until epic to do it