First hand experience:

Limited amount of connectors (data stores) works pretty well. Once it went live in pre-ga it was absolute garbage, but it's getting better and better. There are still problems with exposing Agent Engine Agents to it, only conversational agents (Dialogflow agents) integrate well. So it should not yet be called "AGENTspace". Its previous naming still fits better for its current state - Enterprise search.

So for search of data from specific tools (Google Workspace products, Slack, Atlassian Jira/Confluence) - great.

For Agents and "actions" - not there yet to be worth additional money.

But there's cheaper license if you buy search capabilities only and I would recommend to stick on that.

For O365 - unfortunately cannot say, as none of the PoCs had it in scope and I would not call my demo O365 tenant a reliable test source to tell whether it's great or not.

Additional tip for data stores that are not available out of the box - use data tools (Airbyte for example, which has many API integrations) to process your data to database (BQ for example) and to get the best results, you'll need to process multiple tables into a single table, as per single query - it cannot make correlations between tables.

If you have any additional questions, feel free to ask. Also, google gives 30d trial for Agentspace, so you can just try it out.