I've tried several different blog ad services in the past week, so now I'm fully qualified to make an expert opinion.  And by "expert", I mean "elementary". 

I've noticed that there are several attributes that set each blog ad service apart from the rest.  For me, those are style customization, content customization, payout amount, loading time, and profitability. 

Google Adsense isn't very good with any of these.  Their clunky javascript and forced styling makes customization difficult (though not impossible).  Their content is based on the content of your site, though it's often only related to a single post on your site, making it unrelated to all other content.  The minimum payout amount is $100, which I'm incredibly far from reaching.  Loading time always seems to be pretty long.  I always notice that I wait for pages to load and when I look at the status bar, it says, "Waiting for http://pagead2.googlesyndication.com..."  Profitability also seems to be relatively low.  I've gotten upwards of a whole entire $1 on a single day (wow a whole dollar!), only to fall to $0.10 the next day.  So basically, unless you're running a site with 10,000 visitors a day, AdSense probably isn't your best bet. 
My rating:  1/5 stars

Adster is an interesting alternative because, instead of context ads, they sell advertising space on your blog.  You tell them how much you'd like to get paid, and they try to find you someone willing to pay that amount.  The problem with this is that no one wants to pay to advertise on my blog.  So until someone offers to pay, they display random pay-per-click ads.  The random links are often adult-oriented or gambling related, so I wasn't too pleased.  I just found out that there's an option to disable adult ads, but I don't think they should be enabled to begin with.  So anyway, the style was relatively customizable, the content was relatively customizable, the payout amount was a minimum of $5 (can be changed), load time was ok, and profitability was ok. 
My rating:  3/5 stars

AdBrite is similar to Adster in that it tries to sell advertising space, but in the absence of bidders, displays contextual link ads (based on your choice of context).  The style is extremely customizable.  Content is very customizable, based on certain categories of products or services you'd like to advertise.  Payout amount is set at $25 and can be raised or lowered.  Loading time is good.  Profitability is pretty good. 
My rating:  4/5 stars

Text Link Ads is really nice, but sort of elitist.  Like Adster and AdBrite, they sell advertising space on your blog.  But unlike those two, they don't display ads if no one is paying.  So after a long site approval period and other associated wait times, I'm still left with no ads.  Their code insertion (and optional WordPress or other blogging engine plugin) is quite easy and seems to be customizable.  I don't know if content is customizable.  Payout amount is $25 for a check, and unlimited for PayPal.  I don't know about loading time, and I don't know about profitability.  I'd really like to give these guys a more thorough review, but their system just isn't set up for "little guys" like me.  To their credit, their website is beautiful. 
My rating:  2/5 stars

PayPerPost keeps coming up in my posts because they're the best money-producer I've found.  Instead of displaying ads, they let the blogger write about certain topics and products that an advertiser pays the blogger to write about.  In addition to stupid spam-like advertising such as "Online Shopping with Coupons" and "Wedding Invitations and Stationery", the site also has opportunities to write about things that are relevant to me.  Their blog gives people a sneak peak into the minds of the people who run it.  One blog post suggested that advertisers allow bloggers to be either positive or negative in their reviews, saying, "While I know the concept sounds a bit scary, if you give Posties (our bloggers) an opportunity to be completely open you will get higher quality posts and more posties will take your opportunity."  What a cool concept.  It's also an easy way for publishers to cheaply advertise on blogs.  Their blog has a bunch of other content and behind-the-scenes information about these crazy people and their successful idea.  As for a review:  Style customization is N/A, content customization is great, payout amount has no minimum, loading time is N/A, and profitability is quite high for people who can't make big money with context ads. 
My rating:  5/5 stars

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