I am currently looking at postdoc opportunities in chemistry and was wondering if I would be better off working with an advisor who has tenure. My PhD supervisor only just got tenure and working with an assistant prof has been great. I'm reading though that when you do a postdoc it is better to go with someone who is more established. Can anyone weigh in with their experiences?

First of all, personal chemistry, agreement on working styles and responsibilities, and the same idea about research topics are much more important than your potential supervisor's employment status. In particular the personal chemistry (pun not intended :-)) is enormously important. You must be sure about their personality before going to work with somebody who you do not know in person.

Besides this general remark, from my own experience:

Non-tenured supervisor:
+ More ambitious and proactive, (possibly) open to try new research ideas
+ More presence at university / in the office, possibly easier to reach
+ Possibly better personal relationship and easier to talk to due to smaller age difference
+ Better acquainted with cutting edge of field and modern technology; willingness to use modern communication media
- May travel a lot and thus be absent / hard to reach
- May disappear without warning to take different (tenured?) job elsewhere
- May be overworked (lots of publications, conference committees, grant applications etc.)
- You may be asked to do all sorts of "menial" tasks due to small size of research group
- You may get little chance to teach or present work at conferences, since the supervisor has to build his/her own CV.
- May see you as a threat (competition!) due to working in the same field at similar career stages (beware!): Realize you might end up applying for the same jobs in just a few year's time.
- Mood swings! Rejected job applications / funding may lead to the supervisor being in an extremely bad mood and/or reconsidering his/her career long-term plans entirely. May disappear for months at a time due to this.
- May be worse connected. Possible funding and room / equipment issues.

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Tenured supervisor:
+ Better connected, possibly better funding opportunities.
+ More self-confident and calm.
+ Possibly less short-term work / result pressure.
+ You may have better opportunities to teach - very important for your CV.
+ Ditto for conference presentations.
+ Typically larger research group and better lab equipment mean better opportunities for research. "Menial" tasks are done by younger colleagues or distributed among more people.
+ Supervisor is unlikely to travel a lot and/or leave for another job elsewhere. Exception: sabbaticals!
- Supervisor may be out of touch with field and/or may simply not care about research anymore. You may have to do all the core work / heavy lifting entirely by yourself.
- Supervisor may be frequently absent from university / his office except for lectures and a few important official functions.
- Possibly no interest in developing research group and supporting students / postdocs in any meaningful manner (beyond funding). Group runs on "autopilot" with little interest and little motivation.
- In a larger research group with little supervision: Unclear hierarchies / responsibilities. Infighting may occur!
- Supervisor may be hard to reach (e.g., does not read e-mails and refuses to use other modern communication media).
- Larger age difference may complicate communication / personal relationship.
- You may be expected to shift research areas rather frequently and in entirely different directions due to high-level political considerations (e.g., faculty-level / national / funding). Little understanding for personal research passion or your intention to build a coherent CV.

The question could be "postdoc with a HOD or Editor of a journal?"
I'm not joking.

@Watler_Opera.... you brilliantly sum it up! I am doing a postdoc with a tenured senior prof whom is on sabbatical for the 3rd year and what you described is exactly what my situation is! :)