Hi Yael,
It depends on which carrier you use and the 'connection' speed/bandwidth provided by the radio signal at the time. In a strong signal area using 3G (3rd Generation - with current theoretical bandwidths of up to 7.2MB/sec) you'll find the 'responsiveness' of the browsing experience quite good. The weaker the signal the 'slower' the response will seem, also many other factors will factor, type of websites, internet traffic, server response times, website design and coding etc etc.
Probably the single most 'factor' would be browsing the web on such a small screen - it basically becomes bit of a pain in the but to haveto keep scrolling and resizing etc, newer handsets will have higher resoloution screens and nowadays some form of finger touch control (ie Iphone and HTC handsets) which make zooming into a section of page quicker.
You probably won't do 'all' of your surfing on a small PDA sized device but for frequently visited sites or important work related stuff you can excuse the small screen in order for it to become a useful tool. Away from work etc, you can use it to watch TV and pre-recorded shows and video's etc etc - and the latest devices will and have the ability to stream bbc I player directly ! (some save the content first I think)
Streaming TV will be made into a mass hyped function over the next few months and next year..
don't ditch yer laptop/desktop just yet!
Also, have a look around for reviews on particular handsets and blackberry alternatives - I've just picked up a HTC Touch HD and all I can say is 'wow' ! take a look at the blackberry Bold and Storm as well.. theres some good reviews around!
Hope this helps!
Cheers! DtM! West London & Slough UK!