Whether Vista SP1 is seen as a dud or not depends on what people expect from the service pack. If youre expecting SP1 to mean that Vista will run better on that low-spec system, youre going to be disappointed. If you expect that Microsoft has tweaked the UI and made Vista more like what you want it to be, youre going to find SP1 a huge let-down and are likely to complain about SP1 vociferously and claim that its a dud.
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Others who see a service pack as a way of consolidating the updates that have already been released and as a way to release deeper updates will likely be pleased with the improved stability and reliability that SP1 brings.
If youve been keeping up with Windows Updates and installing the non-security updates that Microsoft has been releasing for Vista youre already going to be reaping some of the benefits that SP1 will bring, because Microsoft has already released several reliability, compatibility and performance updates. SP1 will bring in more updates and more improvements.
If, however, youve held off installing any updates other than security related ones, then youll get all the updates released so far in the SP1 update. Ironically, people whove not been keeping up with downloads are likely to see greater improvements simply because all the updates are all installed together and the results will be more noticeable.
My advice for those wanting to avoid service pack disappointment is to be realistic. Dont expect dramatic changes to the UI, dont expect loads of new utilities, dont expect any kind of measurable performance increase (on the plus side, Vista SP1 doesnt seem to adversely affect performance either). Just expect your Vista experience to be a little better.