Perhaps a little more information about what you are doing would be useful. I am assuming you are creating a 2D platforming game, and that your collisions are between 2D bounding boxes? From what I can see from the code and that information that i've assumed the collisions seem a bit sketchy at best. For example if you draw it out assuming that two boxes of equal size are positioned at the same Y position and are heading to collide in the X axis.

Your collision test (assuming that their position is based from their top left corner) will check to see if the players top left is less than an objects top right position, then check if the players top right is greater than an objects top left position. So if the collision is occouring so that the player is coming from left to right then the collision would work (all though you would want to change the < and > to <= and >= so that you catch it earlier.

So that would trigger the first line in that scenario but as I said if they are equal in the Y position then your second line will never trigger as you again use > and < instead of >= and <=. I dont have XNA so I cant run your demo but I would suggest looking at the actual values being compared within this statement you can clearly see that a collision is happening on screen in order to specificly decide whats going wrong.

As for a more robust technique, why not check to see if any of the four corners of the players tile is within an objects bounding box? That would give you a more accurate indication of when a collision might be happening, as currently if the player was coming from right to left I dont think your test would even trigger the first line (provided that the coordiantes are as I have assumed).

Hope that helps.

This is a rather late reply, sorry. But as for more refined collisions sets, perhaps look into something like Quadtrees, or if you are making a 'screen' based platformer that in itself would act like a form of Quadtree.

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I have been using console.writeline to debug logical errors.. will this cause a peformance hit???
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I'll give you three guesses.

Do a search and replace, and replace all calls to Console.WriteLine with calls to Diagnostics.Debug.WriteLine. In non debug builds, this will be a no-op and you wont get a performance hit.

Anyway, when I ran your game myself, I didnt get any lag, but the little dude fell right through the ground. Perhaps you should do a bit more of the game before you start optimising it?