Hello Jessica,
Many thanks for your interest.
The last Google update ( Aprtil 24th ) was more of a random thing than an assault towards a specific link type. Both thin sites and authority sites were equally punished. So is the case with people with different link profiles. I read many complaints from people whose ranking tanked even though they never built even a single link. So, if similar updates happen, any site can take a hit, regardless of the SEO service they used. But yes, the previous update was more focused towards blog networks and that time sites which got links from ALN kind of networks and HPBLNs were hit.
Answering your other questions :
1. Decent/good initial movement, but then retraction/regression in SERPs' within a week (probably a result of links not sticking, poor quality service, or both).
Any one time package would give only a temporary boost. SEO is an ongoing process and we got to continue building more links to maintain the ranking.
Another important issue could be with where the links are placed. If the service is offering high PR blog posts, chances are high that the blog posts reside on the home page with a PR during the first few days. Google crawls these quickly and gives you a quick boost. But after few days the post with your link will be pushed back and you lose the juice. This exact scenario can happen with social bookmarking services too.
Then of course, issue of links not sticking is there too. But I don't think it has anything to do with the quality of a service.
2. Most of the "complete seo packages" seem to work ok/well for "low-med" competitive keywords, but I have not found them very effective for "high" competitive keywords w/ less than 5,000 exact searches/month.
This depends on what kind of/how much of links a site needs to rank for a competitive keyword. It not just depends on the competition, it will also matter if your site is brand new/aged/number of pages indexed/authority etc. Usually it's much easier for an authority site to rank for a competitive keyword than a brand new thin site.
Here, most link packages, including ours, build a certain number of links for the money you pay, regardless of the keyword competition/niche you operate in. Some keywords will need more, some will need less. But since the price is fixed, always a package delivers more or less the same number of links. This could be a reason why you don't get the same kind of results you expect with competitive keywords.
Thanks!
Tom