A user enters text in a search box and a list of autocomplete options appear. The user clicks on one of these suggestions.
Should they be sent straight to that product's page, or should they be sent to the normal search results page, with that product listed at the top?
That depends on the type of suggestion the autocomplete options are suggesting.
Take Chrome for example. If I type in a term and it is suggesting a search term, which will take me to the search results page, it looks like this:
But if the suggestion is for an actual URL / resource, which will take me to that URL / resource, then it looks like this:
There is a clear differentiation between items that are a search suggestion and those that are a resource. It would waste my time to take me to the search page when I have accepted an item suggestion. At the same time, then only logical place to take me for a search suggestion is the search results page.
So, as long as it is clear what type of suggestion you are making, you should display the most relevant type of page for the type of item being suggested.
As a side note, many services choose to simplify things and only show a search suggestion. That way no matter what you search for, you will always be taken to the search page. Amazon is a good example of this.
It depends on the search box purpose and on the data displayed in the completion popup.
If it's a product search box or if the popup shows a product reference (with an image or other details) it's okay to navigate user to the product page.
Otherwise, you should load a search result page (no exact match found or search is used not only for product search but for a regular text search too).
The point of Autocomplete is generally to bring up the most relevant results immediately; see Wikipedia's search and autocomplete which does this and they bring up specific page names, not search terms, as autocomplete items. If there is a single Item X on your site and you click "Item X" in an autocomplete, generally you should be taken directly to the single page for Item X.
The exception here is where you have more keyword based searches. For Google for instance there's no one Item X page, so instead clicking an autocomplete brings you to the search page for that term. If there's ambiguity or if the autocomplete item is a category (try searching for "electronics" with Amazon's autocomplete), then it should bring you to the search page with that term. Generally speaking the smaller or more focused your site the more likely it is you want to bring them directly to the right page. Wikipedia has direct page results because there is only ever one item with the name of Item X. Amazon isn't quite so lucky as there can be much ambiguity even with a specific product name (imagine searching for an iPad. Which model? Which storage size? 3G?).
