I've took several beta exams (some also from VMware) so I want to spend some words about them.
Beta vs Regular exams
Beta exams usually are for a small period before the run of the regular exams. Usually in the beta exams you have more time, more questions and a way to give feedback.
Also the sign-up procedure could be different and is not possible to re-take a beta exam.
The results of the beta exam are available not after the exam (as it is usual for most regular exam), but some time after the beta period.
Usually a pass in the beta exam could mean the same of a pass in the regular exam... This is not always true (I remember for example the Microsoft 70-113 exam), but in those cases a similar value is used (for example some free vouchers...).
A value for the vendor
The beta exam (some vendor could use a different name) is a way, for the vendor, to make dry-run of the exam to fine-tune the questions, the number of them, how they are simply or difficult, the coverage and distributions, ...
I think that a good exam require also a good beta period (of course this is not enough to have good exam), for this reason beta exams could be a good value for vendors.
For example, to reduce the possibility of cheating (by talking between people or copy by printed notes) you can hope in a good testing center or you can calibrate exam duration to make this not paying... This is a simplistic example, but just to say how a fine tuning could be important.
Good exams can also mean that the related certification is more faithful.
A value for the user
The beta exam could be a value for the user that take the exam:
it's a chance to test himself and the knowledge level
could be a way to see the exam questions before the regular exam (this can make simple to pass the regular exam, in case of a fail of the beta)
could be a way to reach a certification before the regular exam beginning
the exam fee could be smaller (in some cases is free)
it's possible to improve the exam
in some case, could be simple pass the beta exam... (but this depends)
What's wrong with beta exam?
Usually the advantages are enough to say that beta exams could be really good chances both for vendor and users. But there can be some points that can improve some beta exams:
Fee: I agree with some vendor (like VMware) where there is a fee also for beta exam. This could be a way to keep the available seats only for people that really interested. But if there is a fee, the discount rate must be always the same (IMHO I think that a 75% discount could be good )... not change with the exam.
Exam booking: an invitation-only sing-up with individual voucher could be right way to manage the available seats. I've see some exams (I remember the Windows 7) that have filled the available seats after some hours from the beta booking opening.
Exam duration: more that 4 hours could be hard, but I'm the first to say more questions and more time is better. The problem is with some testing centers where you cannot do nothing during an exam (this could be correct... but in a sunny and hot day without cooling... at least drink something could be nice).
Exam modality: the environment must match the regular exam... If something is different this must be notified before the start of the exam. For example I'm usually use the "review" feature to skip questions where I'm loosing too much time or where I'm on doubt... the lack of this function is a mess for me and I have to approach to the exam in a different way.
Exam location: could be correct to use on "trusted" testing centers... But this means that some people have to travel also across countries... I think that VUE and Prometric must instead guarantee a correct quality level of their testing centers... maybe not all... But at least one for each Country...
Beta period: 2-3 weeks are enough. One single week is too shot.
Beta invitation: this must be greater that the beta period, especially when the period is shot... People may have different schedules and exams must be planned both for the exam date, but also for the study period.
Waiting for the results: it could right wait for some months... no problem on it. But I think that beta results must be send before the beginning of the regular exam. This is a big fault of VMware beta exams: for example for the VCP4 results I had to wait several months, with the risk of loose the "window" to upgrade to VCP4 without attended to any courses.
The conclusion is the beta exam are usually good opportunities, there is a cost (and the fee could be only a minimum part, because time is a cost), there can be some issues, but usually is good value.
About the different vendors I think that all can improve the beta exam experience, so I prefer to not judge or compare them.