I don't think the situation with keywords and spaces is all that bad in LR 4. Definitely try the free trial. To summarize the issues:
* LR 3 on Windows did have a serious bug that prevented auto-complete from working properly with keywords containing spaces, but that's been fixed in LR 4. When you're entering keywords to tag photos, LR does prefix match as you'd expect, unaffected by keyword spaces, as explained by dj_paige.
* As Rob Cole pointed out, LR makes it inconvenient to search your photos with exact keyword match, if you have hundreds or thousands of keywords. If you want to exact match two or more keywords, you have to add keyword columns to the filter bar and then laboriously scroll it using its tiny, fussy scroll bars. But the Any Tag plugin makes searching by exact match easier.
* When you're filtering the list of keywords in the Keyword List pane, LR shows keywords in the pane that match any of the words you've typed into the filter box. This can be mildly inconvenient depending on your actual keywords. For example, if you have lots of place names starting with "San" (San Francisco, San Mateo, San Luis Obispo, etc.), and you type "San Mateo", LR will display all the keywords that contain the substring "san" or "mateo" (e.g. "Mateo Bowl", "Susan Smith", "San Francisco").
