My opinion. Although it is best to get links from unique IP's, MANY of the article directories out there are Pants with a capital 'P'.
I done a test and submitted quite alot of unique, well written articles and submitted them to a list of article directories. Some of them the big players such as Ezine Articles, Go Articles, Article Alley etc etc.... and the rest to other ones which still had a decent PR and Alexa rank.
My findings......... the big sites reviewed and approved my articles quickly (within 1 - 2 weeks max).... whilst the others.... 6 - 7 months later.... are still in review and will probably be in review for as long as those sites are online.
Why you ask?! Simply because many many months ago when article marketing became this big craze and the best way to obtain relevent one way links, people jumped on the band wagon, bought a cheap domain name and set up a custom built (cough... ArticleDashboard) article directory. THEN, after a month or two of getting HEAPS of articles to manually approve, most of which were probably spam or sites that did not follow the guidelines......... they just stopped bothering with the site, which has left billions of articles in no mans land waiting to get approved.
My evaluation of article directories: I know of companies that are at the top of page 1 for very competitive niches and have got there MAINLY because they have hundreds of unique articles in the top directories such as Ezine Articles. All the same IP address, but sites like Ezine Articles are an authority site, and all submitted content is reviewed. Google will see regular content added to this site with your links on and give your site a boost in relation to their article content (keywords). The key is gradual AND consistant.
Me... I have now picked maybe 4 - 6 GOOD article directories and am now submitting unique content to them daily. Also, the big directories get crawled ALL the time, and the majority of articles will get indexed. Other Pants directories that have become stagnent will get crawled far less.
That's just my two pence worth as far as article directories are concerned!
Andy