UserControls can be a good option always when you intent re usability, fragmented caching for performance improvements etc. So here it can be number of usercontrols which targets your webpage. Ajax update panels are not a substitute for usercontrols since we can have multiple update panels to load multiple usercontrols depending upon how you need to handle partial updates.
Postbacks will be just like normal as a normal page . If you are loading dynamically some times you have to handle those accordingly. The only limitation is if you want to share it across the applications it cannot be done as we do with custom controls.
You can check the below link which gives an idea about the lifecycle usercontrols and also order of the usercontrol control events fire .
http://blogs.thesitedoctor.co.uk/tim/2006/06/30/Complete+Lifecycle+Of+An+ASPNet+Page+And+Controls.aspx[
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Hope this helps
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