user11903: I plan to hash at around 250-550 gH/s, would it be profitable to mine solo, or would I make more money joining a pool? Are there any unexpected occurrences that I should look out for when mining?  

user2343: As @jtorba pointed out, your chance of finding a block is your_hashrate/total_hashrate, with total_hashrate being currently ~1.2e7 GHash/s. Average time to find a block is 10 minutes (actually somewhat lower, since the difficulty is increasing). 
So, today your mean time for finding a block is between 5 and 11 months (for 550 and 250 GHash/s respectively). 
However, there are other factors:
The distribution of probable times is pretty wide, so it's quite possible to get your first block after several months or even years.
Current trend is that total hashrate triples every 2 month. So, your chance of finding a block would be steadily and rapidly decreasing.
Short verdict: solo minig 500GHash/s is not enough to provide one with reliable mining income. It's not completely out of question if you wish to take risks.

Update: the probability to mine specific amount of blocks during this year when solo-mining with 550 GHash/s. Assuming exponential network growth.
Hashrate growth per 2 weeks | 0 blocks | 1 block | 2 blocks | 3 blocks | 4+ blocks 
----------------------------------------------------------------------------------
                  0%        |    9%    |   22%   |   26%    |   20%    |   23%    
                  5%        |   25%    |   35%   |   24%    |   11%    |    5%    
                 10%        |   39%    |   37%   |   17%    |    5%    |    2%    
                 20%        |   58%    |   32%   |    9%    |    1%    |    0%    

Source code

11903: <p>I plan to hash at around 250-550 gH/s, would it be profitable to mine solo, or would I make more money joining a pool? Are there any unexpected occurrences that I should look out for when mining?  </p>
 2343: <p>As <a href="https://bitcoin.stackexchange.com/a/20056/2343">@jtorba pointed out</a>, your chance of finding a block is <code>your_hashrate/total_hashrate</code>, with <code>total_hashrate</code> being currently ~1.2e7 GHash/s. Average time to find a block is 10 minutes (actually somewhat lower, since the difficulty is increasing). </p>

<p>So, today your mean time for finding a block is between 5 and 11 months (for 550 and 250 GHash/s respectively). </p>

<p>However, there are other factors:</p>

<p>The distribution of probable times is pretty wide, so it's quite possible to get your first block after several months or even years.</p>

<p>Current trend is that total hashrate <a href="http://bitcoin.sipa.be/" rel="nofollow noreferrer"><em>triples every 2 month</em></a>. So, your chance of finding a block would be steadily and rapidly decreasing.</p>

<p><strong>Short verdict</strong>: solo minig 500GHash/s is not enough to provide one with reliable mining income. It's not completely out of question if you wish to take risks.</p>

<hr>

<p>Update: the probability to mine specific amount of blocks during this year when solo-mining with 550 GHash/s. Assuming exponential network growth.</p>

<pre><code>Hashrate growth per 2 weeks | 0 blocks | 1 block | 2 blocks | 3 blocks | 4+ blocks 
----------------------------------------------------------------------------------
                  0%        |    9%    |   22%   |   26%    |   20%    |   23%    
                  5%        |   25%    |   35%   |   24%    |   11%    |    5%    
                 10%        |   39%    |   37%   |   17%    |    5%    |    2%    
                 20%        |   58%    |   32%   |    9%    |    1%    |    0%    
</code></pre>

<p><a href="https://gist.github.com/al42and/8321600" rel="nofollow noreferrer">Source code</a></p>
