The only thing I appreciate about my experience with Scrum was that I could use the hours long "sprint planning" meetings to reflect on my long term goal of never having to sit through another hours long "sprint planning" meeting ever again.
The kinds of issues that I've seen are things like:
* Teams not knowing about some methods that you can use to estimate large numbers of stories quickly - instead of spending N hours doing rounds of planning poker for each one.
* Teams trying to breakdown too much of the backlog at a time rather than relying on keeping distant
* The PO not coming to the meeting with a list of prioritised stories so folk are trying to prioritise and estimate everything at the same time (I'm not saying that prioritisation can't change during the planning meeting - but the PO needs to come prepared IMHO)
* Stories coming in at the wrong level of granularity so that everybody has to spend time breaking stuff down to implementable chunks.
* Teams lacking any kind of definition-of-ready for stories that are at the stage where they can be implemented.
* Teams lacking common agreed definitions of done for stories
... and so on...
(this is for one or two week sprints mind - if folk are running old-school one month sprints then I guess longer sprint planning meetings are unavoidable.)
Googling around that will find you some variant methods that I'm too lazy to dig out ;-) Ping me if you need more.