Definitely solid. They are an authorized reseller of Level 3 and share marketing budget with Level 3 as well, so there is a strong tie between the companies at a high level.
Some commentary in general on resellers: There are a number of "resellers" of Level 3(and obviously, every other backbone) out there today. As with any situation where you are dealing with resellers, financial position, history, management, and relationship/standing with their provider is always important. One must always ask about their commits, whether they have been met, and whether resellers that are selling bandwidth at rates below cost in an effort to hit the next threshold of discounting for them. This "sell volume and profits will come" approach just isn't a model that works and you need to be cautious about candidates for NexGen-style flame-outs.
A number of my clients work with co-location.com for their IP and colo requirements and have nothing but positive things to say. They have been in business for quite some time and management is ethical and straightforward.
While I have not dealt with XtremeBandwidth, I would stay as far away from them as possible. They are ex NexGen employees and NexGen really left a lot of people handing in the wind when they decided not to pay Level(3). My advice, stay away from anyone who was ever associated with NexGen!
Mike @ Xiolink.com http://www.xiolink.com 1-877-4-XIOLINK
Advanced Managed Microsoft Hosting
"Your data... always within reach"
I don't think they (co-location.com or xtremebandwidth.com) provide any management services so you might have trouble getting support if you need it (you can't go directly to level3 if you're going through a reseller, so you should pick a reseller that employs techs and can help you when you need it).