It is still good to buy bitcoin this quarter because bitcoin's market value is still at 6,000$ mark. But if you had bought it when it hits 5,000$ mark several times this year then that would be the best time.
i believe that 6k is not yet the real bottom because when i see several news online , most of them compose of bad news that claims bitcoin had decrease its market capital . other altcoins such as bitcoin cash , zcash , ripple , etc were also falling hard and will continue to go down ( said by a reputable source )
So i guess this is the best year for buying bitcoins and other cryptos because they will probably go down evenly up until we end this year but im pretty sure there will be a major recovery that will happen on the year 2019 .
If bitcoin continues to fall until the end of the year, I think it won't recover in 2019. According to the chart, when bitcoin prices have fallen to the bottom, it will take several years to recover and hit a new record.