Is it alright to use multiple analytics packages (Google, clicky, mixpanel, etc) at the same time on the same site? Would it be more beneficial to just use one package?
However, it doesn't hurt to also have a server-side analytics package that analyses your webserver log(s). These can give you info that you don't get in a client-based one (for instance, info about bots and other crawlers and also stats for things like images views and other files that aren't page hits).
I've yet to find a single third party analytics that comes close to agreeing with my server side statistics. I can't imagine the confusion of trying to reconcile two third party analytics to my local logs.
Its not as bad as it used to be, but google-analytics still slows down pages from time to time. I've yet to meet a service that didn't.
I can't imagine the benefit of using two, but I would venture to say it might be sometimes problematic.
Its a bit of a pain and sometimes time consuming, but reconciling Google Analytics with my local stats usually gives me a very clear picture. I'd recommend trying that first.
It's both common and legitimate to use more than one analytics package, but as previously mentioned, reconciling the (supposedly) same reports is a lot of work, so it's best to either choose complimentary analytics packages, or at the very least work out the differences between the packages, then choose the combination of reports you like best from the packages and don't look at the other reports.
Using complimentary packages is often a good idea - choose a general marketing-information package such as Google Analytics or Coremetrics, and then supplement this with packages for particular purposes. For example, you might need use a heatmap/form analytics product such as ClickTale or CrazyEgg, or specialized tracking for affiliates or advertising, or some form of realtime/lead tracking system - I'm not totally sold on these unless you're both prepared to look at and act on these all the time.
You can also run operations-level packages, to determine server load, bandwidth used, robot traffic, etc - these serve different purposes to the JS-based marketing-info packages such as GA or Coremetrics or Omniture so I think that they should be considered separately.
It depends whether you really need the extra features from a mixture of different packages.
Anything that has a client side payload will erode some speed (and depending on the perspective) privacy for the user.
Generally you will just use one analytics package that has everything you need in it - cross comparing between packages is often difficult so you will normally just stick to one.