Vacation: you're most likely not going to accrue any vacation during that time. My current employer didn't let me start accruing any until I was employed for 90 days...
Health Insurance: Are you currently unemployed? Why would you need to go with Cobra? There are plenty of highly competitive private health insurance options that may be better than Cobra. A lot of staffing agencies will offer benefits, they're just expensive and not the best. If you have a family you may look at jumping on your spouse's insurance for the time being. If you're single... look at the cost of Cobra, private health insurance (get a few quotes obviously), and the benefits offered (if any) by Adecco.
Pretty much every person I placed had the same concerns, and every single one of them were able to continue insurance coverage one way or another with no lapses.
Holidays are tricky, you need to clarify that with Adecco. You're an employee of Adecco if you don't fill out a 1099, so all holidays need to be specified in your employment offer/paperwork. The hiring coordinator or contractor benefits person should explain ALL of this in DETAIL. If you don't understand something, and they don't have the answer: don't sign anything until you have the question answered.
If, for any reason, you don't feel comfortable moving forward, you have the right to request further explanation prior to signing any documents.
Logging time: Every company is different. Some companies have it arranged where you submit an Excel spreadsheet with your manager's signature. Some have you log it online with manager's approval... Without sounding redundant, it's another item that should be clearly explained at the time of hire.
It is important to remember that you may be considered salaried, hourly, or hourly-exempt, etc. Make sure you know whether or not you'll be eligible for paid OT, if so - at what rate?
Get the drift? Just ask plenty of questions and they should take care of you. I understand it's a leap of faith for those who are already "permanent" employees, but these situations typically work out for the best.
*** not to dig at Adecco, but your recruiter should have covered most of this already. I would call them up and inquire about the vacation/holiday/OT, etc. prior to accepting. Once you're in the office to sign paperwork you really don't want to have to review the specifics of your employment status. Signing the offer letter/paperwork should be just that... signing. They will obviously have some procedural info to cover but you should receive/review all this info prior to accepting the job. They should explain the vast bulk of your employment particulars before accepting your... umm... acceptance.

Remeber - Adecco doesn't get paid unless YOU get paid.
[This message has been edited by bkag9824 (edited 5/24/2011 8:04p).]