I'm using 15" Macbook Pro 2.66Ghz/4GB RAM and I'm very happy with it. After 15 years or so spent with MS-based operating systems, it's very refreshing to try and use something new. And OSX is very nice to use and very easy to learn. Of course, I still don't know everything

But I'm able to use most of my usual workflow. I have installed also WinXP (via Parallels virtualization), however, I'm using it only very rarely. Just in cases I want to try something or design some icons and graphics in my favorite Realdraw vector-based editor.
The important thing before purchasing a Mac is to make a list of programs you are using on Win and do a research if there are Mac substitutes. For example, many people say that the OSX Finder is just enough for file management. I personally found it similarly pointless as any other single-pane file manager. And I'm afraid, there is no equally capable file manager on Mac platform as is the Total Commander on Win. Of course, there are some nice multi-tab file managers for OSX, but none of them is as powerful as TC. In the end, I bought ForkLift, which I found good enough and the price was very nice too (I believe I paid 9.90USD?). The total number of programs for Mac is maybe not as high as for Win and most of Mac programs are paid. However, the prices are very reasonable and there can be often bough many interesting and useful programs in program bundles for less than 50-60USD. Bundles usually contain 5-10 apps. I already bought 3 bundles with programs I really wanted and saved many bucks. Of course, not all bundles are useful and worth a purchase.
BTW, 15" MBP is not very portable. If you travel a lot and you consider to take the macbook with you, 13" MBP would be most probably a better choice for you? Especially if you consider to use it with external monitor. I'm using MBP without an external monitor, so I'm glad about the larger screen. But it's not very portable. I often miss something smaller (iPad would be just great

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The only problem I have with MBP is that its power adapter developed a known (and widespread) problem with broken cord near the magnetic connector. But as far as I know, newest Macbooks should be shipped with new Magsafe with redesigned connector. I just bought new Magsafe on ebay, because my existing one is already a pain to use. Sometimes it charges, sometimes it charges not. I heard that Apple replaces the problematic Magsafes on request (even after the warranty). Unfortunately, there is no (official) Apple store in my country so I had no other option than to buy a new one. I hope to exchange my existing Magsafe on my next travel to UK.