I keep mine in a stack. The only real "damage" the boat has sustained as a result of being stored this way is that the bottom paint gets worn off where the boat sits on the forks and the bunks. This really is not a problem since rack storage negates the need for bottom paint. I haven't seen any other damage.
Let me offer some advice that can save you some grief. Before signing up for rack service ask the marina the following questions.
1.) Do they allow you to do your own maintenance (bottom paint, mechanical work, detailing, etc.)?
2.) If "no" to #1, do they make you use a specific mechanic, or can you bring in your own?
3.) Will they allow you to put in on Friday night and tie up to a dock, slip, or pier for the whole weekend?
4.) If "yes" to #3, do you have access to water and power?
5.) Will they put your boat on a "work rack" (or "wash rack") upon request?
Now, find a few of the customers and ask them how well the marina helps them out when the inevitable happens, and the marina either miss your message telling them when to launch, or just plain forgets to launch your boat.
That last question is particularly important. I have been at my marina for one and a half seasons and they missed my message twice telling them that I needed to be launched. On both occasions, one of the owners apologized and launched me immediately upon my arrival. One of the owners lives on the premises and is nearly always there. That is very important. It will happen, and how they deal with it will be a determining factor in your overall level of satisfaction with the service.
I consider myself lucky. We have good owners who in general, let me do pretty darn much anything I like as long as I am not bothering anybody else. I can put in or get pulled out as often as I like, so things like maintenance and detailing have no extra haul out costs associated with them. We have floating docks in a protected cove that I can stay tied up to as often, and for as long as I want.
The service is fine. Just be sure to ask questions before signing at a specific marina.
Michael